Saying "this is a better SMS app" got people on-board and let them "upgrade" to secure messaging. That's why I started using it in the "TextSecure" days.
But, sadly, I agree with Signal's reasoning here. Mixing the two protocols was annoyingly complex. If someone stopped using Signal, messages you sent to them would never arrive - with no notification. And there's no obvious way to "downgrade" to SMS.
I was working on RCS a decade ago. I'm glad to see it is finally getting somewhere - but I'm sad it is at the expense of better and more secure protocols.
But there are threads in their issue tracker where Signal people would disagree with basically functionality of responding to text messages on dual sim devices. "We'll always use the Signal registered number as default".
One part of this is a terrible(but working) protocol, the other is really weird product management where basic needs of the consumer are brushed aside with a "I know best approach" that doesn't really work when you're not in the shoes of apple and have the ability to just replace SMS altogether.
By definition, Signal is not going to have bells and whistles and niche features, to keep the codebase lean and easier to avoid security weaknesses.
If you want an app with many features, use Telegram.
(They did however ship emoji reactions in Signal because most people actually like it and the other apps do have it; curiously missing in Telegram).