Which is not to say that I can't see the rationale, but I didn't know and it surprised me.
The next thing I wondered is how much work it is to maintain two entirely separate messaging systems in one app. How much did this drag them down over the years? I agree with you and hope that it can finally get Signal moving forward, there are so many missing features compared to Wire and, especially, Telegram that it is currently a pretty tough sell to move anyone away from Telegram.
Apple's iMessage/SMS handling is a perfect model for how Signal should have approached SMS support: a firm hand of features and UI reinforcing the fact that SMS sucks, but reasonable enough support that you can send and receive SMS/MMS just fine.