I don't control most of the discord communities I'm in. Some have been going a long time, and every platform migration sheds and shreds members. The 'mild effort' to move an old community to a new platform more often than not killed the community
What's the problem? You're filtering out people who don't really care about participation in whatever group or society is there. People who want to participate will move to an acceptable service and those who feel that is too much effort probably weren't participating much (if at all) anyway - in that case the only difference is the visible list of people with accounts going down, not the actual "users".
It’s also a futile effort since age checks for adult content is becoming the law around the world so soon any platform you move to will have the same checks.
Most people just care about being able to talk to each other, not their devotion to some "group or society".
You underestimate how many people would rather do nothing than be inconvenienced, sadly. If you're not the personality that the community is rotating around, you'll find the migration pretty lonely.
Heck, even esablished personalities can only do so much. Remember that Microsoft paid top Twitch streamers 10s of milllions to move to Mixer for exclusive streaming. Even that wasn't enough to give a leg up.