If Mastodon became as popular as Twitter it would end up with at least part of it being almost the same as Twitter. The main reason for the differences are that very different types of people are using the two platforms currently.

Reddit has backups, so for AI training purposes I'm sure they can restore all the posts of everyone mass editing/deleting this month.

As a teenager I pirated everything, but that was mostly because I didn't have the money to buy it anyway. These days I mostly use FOSS and buy things like games. That being said, my 11yo likes The Sims 4 and it is almost $1200 for everything so fuck EA.

Most if not all 3rd party apps have already put up announcements that they will cease to function on 7/1. I suspect most people will just use the official app if they haven't already abandoned reddit.