The objection to the episode's use of the mycelial network isn't in the possibility of spiritual interpretation, the objection seems to be to the way in which it was presented, in a way that stretches the audiences credulity.
I've not seen the episode since last week, so my memory is hazy.
But, I guess what is being argued is this... that the more contrived and faux-prophetic things are presented as, the more a drama lacks 'social realism', because in the real world, burning bushes do not generally solve Police Detective's cases for them, whether the burning bush is "all in the mind" or not. The principle of Occam's Razor, is that you should limit the number of assumptions in formulating a theory. It isn't an ironclad rule, inviolable, or anything, but in general, is a good idea. What happens when drama starts ignoring statistical likelihood, and justifying increasingly arbitrary things, like pseudo-mythological occurrences, treating psychological states more and more like visits to fae netherworlds from Celtic mythology? Then it perhaps seems more contrived on a sliding scale of audience credulity, or maybe even passes some qualitative border into being a non-naturalistic show. Symbolically, other worlds are often metaphors for psychological states, but they don't actually appear as such in reality.
Modern drama, perhaps being influenced by Lost, is full of horrible contrivance, based in a desire by writers to use tropes from religion and myth. That would never have made it into TOS or TNG. The principle behind TOS was to make a show that could plausibly be accepted as on par with police procedurals and stuff.
I don't know if I'm articulating this very well, I think I'm touching on the issue, but missing some of the details, but I don't really have time to re-watch the episode right now. Star Trek treated everything as natural, and I think that distinction is perhaps important. Also, almost nobody in TV uses the subconscious plausibly, i.e. full of humorous/ridiculous imagery..
Kirk even refers to it at one point as Spock's immortal soul.
Kirk explicitly uses the word 'soul' so I chose to, and there is definitely more than one's 'consciousness' at work. Personality, memories, values and beliefs, can all be transferred from vessel to vessel, or stored while the physical body is regenerated.
I always took TOS's occasional use of words like soul to be poetic licence by the likes of Kirk, McCoy, etc.
Growing up in the UK, all schoolchildren that attend a state school listen to Church of England sermons in assembly each morning. So when I was a kid, I had some vague notion that we were a country with a Christian history, and that the Bible was a source of wisdom for people, but that the age of it being taken literally as a historical or scientific document had passed. Even then, I figured that since souls were unprovable, people largely used them in terms of poetic descriptions of a person's overall essence or personality.
Now, with more exposure to philosophy, and remembering Kirk's apparent familiarity with Spinoza (from "Where No Man Has Gone Before"), I theorise that he may have had beliefs similar to say Albert Einstein, or the deist founding fathers and later presidents of America (Kirk was a Lincoln buff); Baruch Spinoza basically redefined god as a natural/impersonal/non-sentient force, akin to Tao in Chinese philosophy.
This overly emotional objection is not really based on Science so much as Scientism. Mainstream Science has nothing meaningful to say about such experiences because it simply cannot account for them - there is nothing to measure. There is also no generally accepted theory about how such phenomena might work. There are plenty of things that are poorly understood and cannot be measured - at least not reliably - this does not mean that they don't exist only that we, at our present level of understanding, cannot fit it into our models of the universe. That's all you can say about it from the scientific perspective. Saying anything 'spiritual' is 'silly' is not Science - that's simply a metaphysical statement based on a particular belief system.
As Jung said, you can learn a lot about yourself from the images thrown up by your subconscious, which works on a much faster level than the conscious part of the brain, and I see religious phenomena as having a basis in this, as well as nature and secular philosophy. The ethics and metaphysics of the major religions are all presented in much the same way by non-religious, or pagan, philosophers, sometimes centuries before they were appropriated by said religions. Avatars, Rishis, Messiahs, Yogis, Prophets, Sadhus and Buddhas achieved insights through dreams from their subconscious, meditation, mental illness like psychosis, metaphysical reasoning, ethical reasoning, psychological reasoning, and emotional insight. People like Jung, and the people that followed, quite plausibly explained the subconscious, albeit in a "soft science". Neuroscience explains the relationship between mind and body. In the East, humanity was seen as perfectible, and phenomena of the mind seen as natural (in say Taoism, Buddhism, and some forms of Hinduism), but in the West, prophets who may have suffering psychosis (which can entail helpful as well as unhelpful voices from their subconscious), might be taken to be receiving these things from outside themselves, i.e. a god, since there was not such a strong idea that such phenomena could originate within us.
But no human in any reliable record, has ever experienced anything outside of nature; that should tell everyone all they need to know about the paranormal/supernatural; in 150 years of cinema, nobody has recorded a ghost or miracle, and stories that claim the supernatural are inherently unfalsifiable; these concepts cannot be falsified by design, where pretty much everything in nature can be falsified.
I'm spiritual, in a secular sense, of reflecting on my life philosophically.
But I do not believe in gods, afterlives or immortal souls (neither do Buddhists, as an example of a agnostic religion, or possible Taoists). But again, this wasn't really the issue. The issue seems to have been the use of Biblical tropes in more and more genre fiction, when they are a poor fit.
They do for the Prophets and Pah Wraiths, and Kirk (albeit somewhat facetiously) suggests classifying Trelane as a God. But then, to dissect the use of the term, what is a god if not a being of supernatural powers with the ability to create, destroy and alter at will, immortality, omnipotence, or whatever other powers you care to throw at them? Q certainly has nearly all the abilities attributed to the Christian God, for example, if not the values. We may refuse the term, but we are really just renaming the same thing. The Prophets demonstrate great powers of creation and destruction, a simultaneous awareness of all of time (raising huge issues about free will and interpretation of randomness), and the ability to manipulate events in one time (by impregnating a woman with a saviour, no less) to bring about a result they desire in another. They take an active interest in the fortunes of a chosen people, and the show is very open about the uncomfortable distinction between gods and wormhole aliens that Starfleet is trying to draw. The show's lead character definitely falls on the 'believer' side of the equation by series end. Now you may not like that, but it is Star Trek, so the assertion that Star Trek has been entirely rational and natural on these matters simply doesn't wash. Trek is a universe of gods and magic, it just pretends they are scientific.
Respectfully, I disagree.
A god in the Christian/Muslim sence, is most often interpreted as something outside of nature. For all their powers, and for all their literary similarity to gods in fiction, the aliens of Star Trek have never been treated or presented as actual gods are in these middle eastern religions; they are within nature, and comprehensible to scrutiny. They have always been presented as part of the natural world. And that is an important distinction, I feel. It has massive philosophical repercussions.