People act like Gmail came in and afterwards there were no more email providers. Did a lot of people use it? Yes. Did it somehow kill off all off the small email providers? No. Some of them? Sure. All? No.
Why did I switch from Gmail to Proton? Because I stopped trusting Gmail. And felt that I could trust Proton more. Can I still email someone that uses Gmail? Yes. Do I have to support Gmail? No.
The purpose of an open protocol is choice. Otherwise why bother making it open?