My personal predictions:

Reddit will continue to exist, but become an increasingly zombified dumping ground for screenshots from other platforms, videos of people being injured/humiliated in public, and lowest-common-denominator "discussion" of culture war topics. It may also to continue supporting small niche communities like r/HVACadvice where you can get a quick answer about something specific from a domain expert, but will largely just become a firehose of diarrhea.

Large Lemmy instances like lemmy.world will pick up the slack for the mostly good-natured people turned off by the advertising and shitshow at reddit and other corpo social medias. It'll be better than current reddit, but increasingly enlightenened centrist, and importing braindead redditisms (shit like "and my axe" etc.)

Smaller Lemmy instances that care more about the kindness and quality of discussion will pour more grassroots efforts into strict moderation, and defederating from mainstream instances. They will tradeoff a wider userbase for a more kind and nurturing atmosphere.