I'm running a 3rd party pkpass app on Android and it works smoothly. Only 2 clicks to show a pass.
LNER (and I think now Avanti) support Google Wallet for storing tickets but not Google Pay for purchasing them.
If a search engine doesn't autofill then everybody has to fill in month/day/time. If it does autofill to today then users travelling today only have to change the time (faster), and other users have to change day/time (faster) or month/day/time (same). So logically, autofilling does make sense.
I've just used Avanti's and it has a very similar visual layout and design to GWR's. Both are inferior to National Rail Enquiries, in my view; I rather hope NRE doesn't change to a GWR or Avanti style layout. It's not as clear-cut as you're making it out to be.
Have you tried recently? The booking flow order is quite different. For example
GWR: Return field hidden. Need to click a button to display it.
Avanti: Return field always displayed. If left blank it assumes you want a single.
Small differences like this add up along the booking process meaning the difference between clicking 15 buttons to purchase, or far fewer.