This is the one I'd be worried about. Thought it was annoying to not be able to use banking apps on a rooted Android? Think about how annoying it will be when you can't do much of anything, even on the Web, unless it's from a sealed, signed Apple/Google/Microsoft image-based OS...
I realize the way Firefox's user share is going, it might not matter or they might feel they don't have a choice but I really, really hope Mozilla doesn't even remotely consider implementing this.
Online fraud and theft is exploding right now and the average person is simply not capable of securing a laptop so the companies have decided to only allow secure access through a phone which can usually be trusted to be malware free.
It has to stop somewhere. 100% security may reduce the banks' fraud costs but it isn't acceptable for personal freedom. "Choose a different bank then" only works until all they all adopt it.
I assume an old person cares about not being left poor and helpless in retirement more than they care about free software and computing freedom.
I think it's probably likely that we will end up in a situation where some devices like phones and maybe laptops are considered "secure environments" where banking transactions and such can be safely executed, while alternative devices will be available for complete freedom and tinkering. You'll likely always be able to run any program you want on your laptop but those programs will be limited to their own sandbox rather than having free access to any other programs data.
this alternative will basically not exist for all intents and purposes if the "secure" version is the norm.
Let's take an existing example - why is there no such an alternative for home gaming consoles like Xbox or PS5?