I dropped Zwift after the price rise. I have tried MyWhoosh once and IndieVelo a couple of times. They both seemed like adequate replacements in terms of the technology. MyWhoosh was a bit slicker for pairing, in my experience. Smaller numbers of users and events on both, I think, but I probably got a more competitive event in the small group I rode with on IndieVelo, compared to Zwift.
The only missing thing I would like to see on IndieVelo is training programmes, like the Zwift 12 week FTP builder. I think MyWhoosh may have some, but I need to spend a bit more time on it.
I think the key thing is going to be whether either of them reaches critical mass and has a big enough user base to really get established and sustain themselves (and what their revenue solution is).