I'm going to say that Big Tech hasn't devalued online services. They've always been devalued. In the days of BBSes, most had both paid and free options. The free users might get bumped when the board was busy but that was it. You were often still able to dial in at all times and do everything on the board.
Honestly, that concept just carried forward. Usenet access was simply part of your internet access payment to your ISP. Free to join. You just needed a free NTP app. Many web forums were the same - completely free to use and maybe there were some ads, donations, or whatnot to help fund the site. Some even used forums as loss leaders - Harmony Central had their main page along with forums. Ditto other sites like Something Awful and Bodybuilding.com.
Right now the short-term is a mix. The reality is that the major jumps have been to Discord, not other forums, or so it seems. Subreddits already have their own Discord servers in many cases and there's a load of them on top of that. Beyond that, it seems a mixed bag of people trying to find a new home. Talks have varied outside of the Lemmy / Tildes mentions; Fark, Hacker News, City-Data, and various smaller forums.
Seems we'll be split on to at least a few if not many disparate forums and sites to fill various needs.
As someone else said, Reddit will die a death of a thousand cuts. The big hits will be the 3rd party apps loss followed by the expected old.reddit.com loss. That said, the majority of users don't use either of those. I'm seeing some general large site / social media fatigue combined with a lot of mentions of Discord. Given they've even got forum channels now, it seems Discord may be one of a few new smaller web forum options like we had in the late early 2000s with the start of software like vBulletin and phpBB. It's not looking like the Digg to Reddit migration. It's more the Usenet to web forums migration. Reddit is the Usenet 2.0. We're now scattering and waiting for the Usenet 3.0 contenders before a new champion is crowned.