My primary issues with the spore drive have to do with it's incredible ability to instantaneously transport you anywhere in the galaxy (or even universe maybe?) and that it was a central crux of the entire show. In story, with the drive that can do what it does I don't think Starfleet would so easily abandon it. It damages the multi-verse, yes. But I would think they would have teams of scientists finding ways to make it a safe technology. It might take years but eventually, knowing how Starfleet is generally depicted, they would find workarounds that make it safe. Now, Star Trek is indeed littered with the remnants of technologies that are here today and gone tomorrow. But that's my second point. It's a little easier to ignore something that was only used once in a single episode or movie (levitating boots anyone) that never show up again. If spore drive was just a one episode thing then it might just be the next in a long list of technologies in Star Trek that goes bye bye. They compounded it by continuing to use it in 2 seasons (maybe more if it's still used in season 3). And it even became a crux of the show. Eventually you'd think some smart spy would get hold of it and sell it to the highest bidder, possibly to a power that holds different ethical standards that might not care the damage it does if it means they could defeat the hated Federation. Now all that being said, Discovery did add in another in-story reason for it's disappearance and the fact that we know it is never mentioned again with classifying it. I still think it's a bit clumsy, a secret that large involving such an incredible technology would be extremely hard to keep under wraps. But I'll give them points for at least trying to address why spore drive is never heard from again and why Starfleet never tried to work on fixing what was wrong with it.