You say this but Matrix is largely centralized so it would be easy to get the biggest node to comply. Servers are quite costly to run too which is a big problem.
I don't think Startmail will be affected. Ofcourse using Gmail is free pass to your data. But look at this https://www.startmail.com/ . I think if you also use Proton or other mailing services you're 99,9% safe. I sometimes play video games, some of my friends are kids who are cursing in gamechats. How will gaming chats be affected? Does the government have access already?
Why are they still thinking they can justify this? People which do illegal stuff are just going to use old apps self-host or bypass it with another way.
Of course there are alternatives. That's the cherry on top of this crap pile: only regular non tech folks are affected. Nerds and actual criminals will just run an xmpp or simplex server and not care about the legislation.