Matrix user with bridges here
I'll add my thoughts to the conversation as I've been using (self-hosted) Matrix services for a few years, and recently added the whatsapp bridge.
I decided to switch to Matrix because I really liked the e2ee thing. Also, at the time the video call quality was so much better than skype or whatsapp (as you would imagine with a dedicated server and absolutely no bandwith/quality throttling - I once went through 500mb of mobile data in a few minutes).
Yes, I still use whatsapp, Matrix is only between me, my wife and my parents (which ARE grandparents now, so yes, grampa is on Matrix).
Do I have to pay for this? yes, in the form of hosting (TURN does not work behind a nat and so can't self host at home). And it's pretty much the equivalent of $5/month. I still do it because a) it's not much really b) I like it and most important c) I control the data, ALL of it (well, the one in Matrix anyway).
Recently I've added the whatsapp bridge, as I was tired (read: too lazy) to have both the Element client and Whatsapp client on my computer and having to switch between the two. And since the only people that I have video calls with are my wife or my parents, the lack of that integration is not much of a bother. Yes, it will be nice when it comes so I can call my sister too (I am not a monster, I use a predefined jitsi room with her).
And now whatsapp is allowing to share up to 4 devices, so I think I'll try to virtualise android and let whatsapp run in there instead of my mobile. Not sure it will work (actually I am pretty sure I understood wrong what whatsapp means, but being wrong never stopped me from trying - that's how I got married, I think).
Maybe I'll add the FB bridge as well so my wife can use it, and maybe the signal one for the two people I have in there (and actively use it)
Will whatsapp and FB make breaking changes in the future? Most likely, but not suddenly as there are a lot of businesses using those services, so they will be announced, and I won't be the one who has to implement them, I only need to remember to do a docker pull when it's ready.
What' the point of all my rambling? Well, not much really. But a) I would _maybe_ consider switching to a managed services if it was cheaper b) there are people/use cases for such a thing and c) I got you hooked up to this point, didn't I?
Cheers!