As a user I don’t want these other apps getting access to my incoming SMS messages. I’m fine with one app getting all the messages. It really doesn’t make any difference whether my messages have green or blue bubbles (and people complaining about that are really stretching).
As for the argument that 3rd parties should be allowed access to the iMessage network. When are all those other apps coming aboard that plan? When can I write my own app for Messenger instead using Meta’s?