> when you can just open the thing in a website anyway. I can use my bank on some linux distro

Unfortunately not.

I'm in the UK. Two of my personal banks, all four business banks that I need to use, and several credit cards, require authentication using their phone app to confirm login on their website.

None of those I've seen are using TOTP or SMS, for which I could use a general security service. All use their own phone or tablet app. One does something interesting where the website shows a unique QR code on each login, the phone app reads it with the phone camera, and then website login proceeds instantly without clicking anything.

Oh, and some of them also require phone app confirmation for card purchase transactions.

When my last phone's screen stopped working, I called one bank's "phone banking" line (using another phone of course) to make an urgent transaction, and they told me they can't do that, as only service they offer by phone is registering a new phone or tablet. They told me explicitly that it's not possible to login to their web-based banking service without using their app for authentication, and on a registered device.

It's the reason I have my current phone. I had to buy a cheap-ish Android in a hurry from a local shop, in order to proceed with my bank transaction.

Back to the main topic: I love the idea of a properly open source phone, I used to own not one but two Nokia N900s, and I once toyed with the idea of building my own Linux phone from scratch, big project though that is.

But the security ecosystem around logins has changed, and so have the services I depend on. These days I use many bank and other financial-service related apps, and I'm not, in practice, free to switch providers. So I couldn't use a Nokia N900 or modern equivalent any more as my only mobile device. I'd have to carry a second phone as well.

(Banking and other service authentications are also the only reason I have my current passport. I resented having to pay to renew my expired passport, given I had no plans to travel (small children) and the expired passport used to be accepted, but I found some banks, credit cards and even government services increasingly requiring to see a non-expired passport from time to time. When I asked one of them what do they do for the large number of people who don't have one, they simply told me they close those people's accounts and that's ok, they don't need to serve everyone. But that's another story.)

> require authentication using their phone app

And banks often have their apps region locked, so if you live abroad or have accounts in more than one country, you’re fucked.