people on new phone are now auto enrolled to Google's RCS. This poses a few issues:

• if a user switches from RCS to SMS they're now using a less secure messaging mechanism.
• it's confusing (to some not all) to have a very insecure method of messaging in a secure messaging app.
• if a user switches from RCS to SMS and they don't know to unregister, they'll now be sent RCS messages that never arrive making Signal look like the cause of the problem.
• Google hasn't provided an Android API for RCS. the writing is on the wall, text messaging via alternative apps is no longer a long term option and a waste of developer resources when they could instead be improving what they do control, Signal.
• SMS on Signal was already unmaintained and not that great.
• SMS was a feature on a single platform, Android, and didn't even sync to the desktop app, confusing some (not all) users.

All that said, with limited resources and budget, they really should not be spending any dev points on 40 year old tech. does it suck for some? yes, absolutely. Is it the end of the world?

Lastly, while I love Matrix, its not ready for prime time. I was able to get my grandma to install Signal and join group chats with almost no help. I wouldn't fare as well with Matrix/Element.