Why build a first party messaging app if most people aren't going to use it? For people who just want to send plain text messages the UI is pretty much identical. Text box, type, send.

What if I want to receive plain text messages?

Turn off iMessage or tell your friends. Or get over it because it's just a silly message?

Use email instead. Or turn off iMessage and just use SMS.

In that case, I think I still have an old Motorola Razr lying around in a drawer somewhere you can have...

Did you miss the part where they indicated that Messages was the most used app on iOS? Why would they abandon that experience for their customers and leave it up to a 3rd party to control?