Apple seems to maintain iMessage and SMS in one app just fine. Granted, they're a bigger company, but it's hard to say just how large the Messages development team is. It could conceivably be even smaller than Signal's workforce!
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.