I have 5+ year old Android phones that have no issue with banking apps. I'm not sure where this rhetoric is coming from, but it doesn't align with my experience on Android.
Capital one/wells fargo requires android 8+. Citi bank 7.1+. (I stopped looking it up at this point). 8 was released in 2017. So assuming 2 years of major OS updates, that means roughly phones from 2015 can still work with it.
Also... even if the app stops working, you can use a web browser still.
Phone batteries die off long before this becomes a reality.