iPhone fans by definition would not be a fan of the Google OS at the very least, but you think they would not go with the apple arcade game service and instead go for the service made by the developer of Android OS?
This is like suggesting that someone with a PS4 might get into games at the point where Halo comes to the system.
The only thing they need to do is download Chrome for iOS, and Stadia will be working through Chrome. Until eventyally Google comes with a proper Stadia app.
So.... open chrome browser, navigate to stadia, open Doom Eternal that you paid 60e for in the app.... and..... touchscreen controls for Doom Eternal?
That player doesn't exist.... if you wanted a portable version of Doom Eternal and you're fine with paying 60e for the game you would get a Switch Lite and copy of the game that way, it would be infinitely more playable than using a iphone with no controller.
I know there is controllers you can attach to the phone if you want to get into gaming. but you are suggesting all they need to down is download the app/browser, that experience is going to be DS without further investment.