Not useless. It is like the missing printer driver for Linux Desktop. It makes the experience ugly, but this is not the fault of the Linux OSes.

Also the bank should not require apps (instead they can offer hardware key support or desktop apps) and in fact some - at least in Germany - offer a different authentication possibility. Also the app for the German ID is published on fdroid and does not rely on Google services.

Good for Germans then. Slovenian banks won't let you use physical 2FA authenticators (for personal accounts and maybe even business ones at this point) anymore and will also require you to constantly update their stupid app (I've had to replace some otherwise good phones because the OS version wasn't supported anymore).

There are plenty of banks in Germany which offer over-the-counter services, if you prefer to do banking as if it's 1999. Most of the time, when people say it's impossible to live without a smartphone, it's actually only impossible to enjoy the conveniences of the internet without a smartphone (at least in Germany). Besides these rentable scooters, I can't think of anything that actually requires a smartphone. Sure, you'll miss out on a lot of conveniences, but I remember a time where that was the norm, so it's not like it's unreasonable.

The question of how useful or not it is is orthogonal to whether it is the "fault" of Linux. Users who can't use it because something they need just doesn't work won't change their minds because the blame lies elsewhere.