1) so long as these age verification methods are for things at home, I think most people opposed to smartphones ("client devices" we have to pay for) will not be too inconvenienced. a few years ago, broker quit letting us use plastic code cards for 2FA and required we used an Android/iOS device, but I'm willing to tolerate having a single-purpose tablet in a drawer, though I'm still annoyed to essentially have a $100 undisclosed fee attached to service I have to pay every 3-4 years when they drop support for older versions of Android.
2) I've so far only been locked out of minor services like Anthropic by not having smartphone w/ mobile service, not because they require app use but because they ban VOIP numbers for registration. many businesses do charge me a little extra for not using a smartphone, but the cost is not too high, about on par with ignoring loyalty cards.
3) this cannot ever be required for something like onsite ID verification (e.g. at an airport) or I would be locked out of essential government processes. I'm willing to keep a device in a drawer, but will never carry a client device around with me.
4) this is not a problem exclusively for elderly grandma with satellite brick phones. I'm in my 30s with IT/developer background.