I agree with both sides of the argument. While Chase and BoA makes plenty of money, you also have to remember that many things are compartmentalized. Microsoft makes billions every year but that didn't stop them from dropping Zune, dropping KIN and dropping numerous other products throughout the years. Same with Google. Eventually some department manager has to justify why they're pouring money into a losing platform. So while Chase/BoA have plenty in the... bank, someone has to justify why they're wasting resources on a platform with so few users. Someone must of did the math and decided it wasn't worth keeping the app alive.
However, it does seem shortsighted given what we know, and likely what Chase/BoA also knows, that a unified Windows will open them up to a new range of possibilities. There are plenty of Windows devices out there. Hundreds of millions of Windows 7/8 users that will soon have Windows 10 and access to thousands of apps across a range of devices, be it desktop, tablet or phone. That includes millions of businesses/consumers that could start using the Chase/BoA app. And just like how people aren't dropping their current devices en masse, there won't be a mass exodus from Windows to OSX either. It's hard to convince people to buy whole new hardware (and sometimes platform specific software). So when consumers need a mobile banking solution, and don't want to replace their current desktop/tablet/phone, they're going to find a solution that works.
That's why Wells Fargo was smart to advertise their continued support for WP. They understand that in the long run, being everywhere for everyone is more important than a few bucks lost on a mobile app. It's the same reason Microsoft continues to support WP, it's the same reason they're adopting a unified strategy, because in the end they want to be the company consumers turn to no matter what device they're using.