I have a hard time believing that about plenty of other technologies, as I've said. I have a hard time believing they don't use transporters as weapons (just turn off the rematerialization part and it's a disintegrator ray that can shoot through walls without line of sight, basically the Tantalus Device) or as a replacement for surgery. I have a hard time believing they didn't keep the Kelvan drive modifications and make their ships a hundred times faster. I have a hard time believing they haven't combined the quick-cloning from "A Man Alone" with the mind-transfer tech from "The Passenger" to make everyone immortal. Things that are hard to believe about Trek tech are hardly exclusive to the newest incarnation.
Yes, but my point is most of those were one episode devices that appeared and then were never seen again. The spore drive is not something that made a single appearance and was forgotten about. It's a major plot device of the entire series. When I think of Discovery, spore drive is one thing that immediately comes to mind, like warp drive for the original series. It's a critical part of the show that if you removed it, the show would be changed substantially (just like if you removed warp drive from the original series). If you removed the Kelvan drive you might lose a single episode, but it's not a crux for the entire series (and therefore easier to explain....maybe the Kelvan's simply removed and destroyed the tech claiming it as proprietary, just for instance).
I'm not sure how you'd effectively use transporters as a weapon though. All a ship has to do is raise its shields and that effectively stops its use against other ships. I suppose if you knock their shields out you could use it to transport weapons but then, if you knock their shields out you can just use your ship's weapons to do the same thing (that would seem to be the simplest). And against planets it still seems to me it would simply be easier to use your ships weapons. It could make for a good stealth type weapon I suppose against a specific target. I suppose you could argue its usefulness in surgery, but it's not something I thought of much. It seemed at the end of the day the way they did surgery was very non-invasive and effective already. Perhaps they already use some form of transporter technology in medicine (like we're told it forms the basis of replicator technology and holographic technology).