I don't think that many content posters will leave. Sure, in tech oriented communities they will, as they are the ones most receptive to fediverse or other alternatives.
But painters, photographers, historians, chefs... etc are a large part of what make reddit great. And plenty of those don't really give a fuck about the platform. They will just use the official app and move on.
Yes! That's what happened with Facebook too. Techy people left the platform long ago but it's still really useful for other types of users, the ones who don't really want to invest time into learning how to use a different platform.
I deleted my account around 10 years ago, but my SO is still using hers and it brings her real value. She is active in the local communities, uses the marketplace, etc.
Ironically, I've been trying to get her to use Reddit more over the past few years...
My SO also gets a lot of great travel and cooking tips from Tik Tok. She only recently made her reddit account lol. But that is for a medical forum, so also one that I dont expect to transition. Luckily for me, almost everything I used to follow was techy in some way, so I'm not bothered by the difference in content over here.
For most of them this is just a minor inconvenience compared to the gigantic community they get.
Im sure if reddit keeps this path it will deteriorate and lose users, especially as new generations don't enter it and prefer other platforms, but it still has a very long way to go.
Even twitter, which is full-on scorched earth with big messes like limited viewership, still retains a large active community.