You seem fine making lots of assumptions when they justify your position?
But even ignoring all those assumptions, in the comparison you wrote, Stadia Pro comes out wayyyyy ahead. There's no way I'm going to buy a PS4 Pro at this point at that price, it's way too expensive for the limited amount of time that's left before something better comes out. I'll end up paying over $400 over the next 18 months and then I'll want something new.
That's really my only important point. With a console there's risk because you pay for all the expensive hardware upfront, and the hardware never gets better (sometimes it breaks and you need to buy it again if you're out of warranty). With Stadia you pay for the hardware monthly, so even if it's $120/year it starts off much cheaper and it takes awhile for the cost to break even with other options.