I'm in tech, sports, and social Meetup groups. I don't want to join 3 new sites to replace this one.
You do not need to have a single platform, though. All you would need is a single, in the best case open, index of meetup sites and descriptions. For that to work, you would need a simple protocol, e.g. myownmeetup.com/meetup.json, which the aggregators can crawl.
One of the things I liked about meetup was that many (most?) groups were hosted on it, but I think what this has shown is that meetup had a kind of monopoly in that way (which wasn't good!)
Create an API that allows any site to be a front-end for meetups, you just store the data. Not sure how willing other sites would be to use this though.
Mobilizon supports federation.
Really though I don't think Meetup is going to die. WeWork is selling it, there's no way the market would just let this huge captive audience of active users implode. Maybe FB wil buy it.