The end of the finale felt like the culmination of an attitude towards writing that I really hate. Small Universe Syndrome is one part of it, but it goes far beyond that. In being desperate to tie everything back to TOS, rather than just using the setting and time period as a vehicle to tell new stories, they've made it feel like Starfleet has about three ships, Spock is the most important person who will ever live, and the alpha quadrant is about 5 miles wide.
It's almost the exact opposite of the feeling TOS evokes, that the galaxy is so huge and populated by so many civilisations and insane cosmic entities that you can fly off in any direction and find something nobody else has ever seen before.
When the first rumors about a new series were coming out, my best hope was that it would be a JJ-verse adaptation of the Vanguard novel miniseries. Book adaptations were (and are) popular, it did a great job taking advantage of what made the 23rd century unique as a Star Trek setting while broadening and deepening it, and tying it in with the movie franchise would allow them to get their
USS Enterprise cross-promotion and maybe borrow some sets and costumes now and again.
Two seasons in, any prospect of that has fallen away. The TOS tie-ins are superficial and often nonsensical (Let's reference the old BTS idea that Number One is her actual name because she's the best on her home planet at everything in the same episode we show she doesn't understand geometry), the broader strokes are totally ignored (the Klingons seem awfully chummy for people who were at war with the Federation last year, and will be at war again in ten years), and the best solution the writers can think of to the fact that they can't make the TOS era interesting is to invent a totally new era that, spoiler alert, they'll probably also fail to make interesting, because the problem is not the calendar.
I mentioned it earlier, but, it was stated on two separate occasions by Stamets about how first; Starfleet was searching for a non-human interface to operate it. He later says that it was decommissioned. My point being that even thought it isn't EXPLICITLY stated that Starfleet stopped using it, if you left it at that, it would be easy to just fill in the blank and assume Starfleet didn't succeed in finding an alternative way of operating it, so, the technology was abandoned.
We probably didn't even need that much. They found one giant tardigrade in all of space, and now it's gone. Likewise, Stamets' operation on himself seems to be impossible to replicate (at least, without another tardigrade), so Starfleet can have, at most, one spore-drive ship, and that's only as long as Paul Stamets is alive. You can fill in more details, but the upshot is always going to be the same;
Discovery is unique. Why didn't
Voyager build a spore drive to get back home faster? The same reason they didn't build a copy of Data to invent something that'd get them home faster; they couldn't.
Granted, there could be some interesting drama in playing that out. Some kind of "Measure of a Man" thing where they try to draft Stamets into staying on
Discovery permanently because it's an irreplaceable resource, or helping another ship trying to lure out tardigrades for examination only for it to end in disaster, but DSC has made it clear it doesn't have time to develop its ideas. For a show that made a "Actually, there are bad ideas in brainstorming" joke, they seem absolutely incapable of not putting every thought they can think of on-screen.