And right now, my phone is not usable even if the battery was still good because the banking apps now refuse to operate due to the security environment being depreciated.
That said, the phone is definitely usable - as a phone and probably for texts and many other apps. But if the 2-4 year lifespan of your banking app is the issue, maybe just use it on the desktop or through a browser instead of the dedicated app. As long as you can get a recent-ish browser installed on your phone, you might be able to use the web banking interface (not all banks have mobile-friendly versions, admittedly).
And yes, I believe the official stance of Google with regards to updates is 2 years of updates followed by 2 more years of security-only updates. Other manufacturers may do less than that, which is one reason I get the Pixel phones.
So I think the actual physical performance of the phone is more of an issue than the OS.