Centralization. Meta will develop the fediverse protocol up with their own specific enhancements, locking most of us onto their version of the platform due to convenience given enough time. (Think RCS as an example, it's technically an open standard but Google has effective control of it thanks to special features)
You might say "Well, if they are a better platform, so what, why not go there", but the problem is that they can cut off the rest of the fediverse as "outdated" essentially privatizing everybody.
You can use the Web Archive, if those posts are saved, you'll see it without ever touching a reddit server. You can't comment or give awards or anything, but you're not locked out completely.
Or you can use a 3rd party app. The team behind Youtube Revanced have patched most of the major (Android) ones to still function afterwards. https://web.archive.org/web/20230705025421/https://www.reddit.com/r/AfterVanced/comments/14m3wgo/reddit_apps_for_which_patches_andor_patched/