a) It took a LOT to shift my entire network over to Signal. The vast majority would not want to move again so quickly.
b) I'm not in the UK, and I'm located somewhere where there are several jurisdictional roadblocks to cryptocurrencies. It may not even see the light of day here. This makes it even more difficult to advocate for a move, and also might mean it's not a cause for concern.
I get that this could well be the first in a series of bad moves that are user-hostile, and the centralized nature of the app and the fact that they do not allow third party clients or federation means that you're stuck with whatever they serve you, but I don't want to switch yet.
The number of viable alternatives is just very less.
XMPP is pretty good, but I am yet to find an XMPP messenger that does @ mentions, a feature I and all my contacts constantly use in large group chats, and media delivery has been somewhat of an issue with some XMPP messengers I have tried.
Matrix is slow. I don't have the hardware to self-host it, and matrix.org on Element can be slow even now when adoption is rather low. If enough people move to it, performance will be even worse than Signal.
Telegram security is nowhere near comparable and it has even more bloat and nonsense features I want nothing to do with, although the UX is pretty great. Plus they also have cryptocurrency on their platform, and ads in some shape/form. Wickr etc are proprietary, so that leaves Wire, Threema and Session. They seem alright
https://xmpp.org/extensions/inbox/jid-mention.html
XEP-0372 (your second link) is a way for sending clients to more explicitly mark mentions in a message. Clients are gradually switching to this instead of the text scanning technique.
The first link you gave is to a proposal in the inbox (it wasn't accepted), and the third to a relatively new proposal that is mostly for servers to help them know when to send push notifications to offline users in a group.