Of course they all support side loading and alternative app stores, so you aren't locked in to Google Play/Apple Store.
For now. Goggle is trying to lock down the sideloading and the play store.
The users aren't really abandoning Reddit. A two-day blackout isn't abandonment or a boycott. All you're doing is telling the company that you're so much of an addict that you'll be back in a few days.
A warning shot does not mean you will miss with the next one.
If Lemmy or Mastodon go evil, people still have to find another website and move their accounts and communities over, no different from what is happening with Reddit right now.
No and no... First, Lemmy can not "go evil." Lemmy.ml can go evil. And people can chose not to federate with them. But there are lots of lemmy servers. And mastodon servers. And kbin... It is more like email. Yes, lemmy.ml is the gmail of the lemmy ecosystem, but you do not need a gmail account to send email. And yes, you need to pick you home carefully. Like email. And you need to pick the home for your group carefully as well. But you can, and they can move and it still works.
If it has syntax, it isn't user friendly.