And from what I've seen, the core devs have always supported and encouraged more instances to be created so that there's a diversity of communities ... I don't think want everyone to be just on here (lemmy.ml) and I'd guess they especially don't want to conflicts to erupt over communism (where in the past some facist or neo-nazi brigading happened and that's why sign-ups require approval).
The answer is for some people to get to work and put up new instances. That's what happened at mastodon and it's what allowed the platform to absorb the twitter migration. We really shouldn't expect whole new open-source and free platforms to just be waiting for us to get tired of our corporate for-profit big-social-platforms. It takes a little bit of work from us ... either understanding a little bit about how things work, helping others, engaging, and if we're able, putting up instances, starting communities and contributing back to the source code.