Kbin.Social (rip) and Lemm.ee (rip #2) posts still appear on Lemmy.World. Also pondercat (rip #3) was a bot-only instance (in its case, for tracking RSS feeds). So all in all, I think it's fairly possible, with only minor (?) issues being in the way, like mirroring external content that would be loaded seemlessly (e.g. Imgur on Lemmy and Peertube on Mastodon) and storage and processing power for tracking the instances.