Google isn't a company that restricts their services in such a coupled manner. I fully expect it to be supported on most devices where it's possible to support it.

In my opinion, restrictive would be keeping Stadia to only Google specific hardware. Making it available for all Android devices feels pretty good to me.

I would actually suspect more of the opposite. Their biggest hurdle, at least initially, is fighting the perception that the service just won't be performant enough to be a good experience. I would not be surprised if they start with a limited set of high-end devices (Apple ones included), just so people don't blame device limitations on the service instead.

I don't think so. Google has announced it would be coming to more phones and devices in the future and they consistently say "tablets" instead of ChromeOS tablets. It's a service, it would be silly to ignore literally a billion possible customers (or whatever the iOS numbers are now).

Considering that iOS users, on average, pay a lot more for software, games and services, it’d be stupid for Google to ignore a paying customer base. In any case, Google will not ignore iOS, like it hasn’t done so far in a big way.

There are things that were released on iOS first by Google, like the Gboard keyboard, for example. So Google is not at all averse to iOS or in the habit of avoiding it in any way.