Well, Android without the Google gubbins would be open in a way that the Google gubbins ain’t - which is why I suggested Amazon Appstore, not that I’m a fan of Amazon (far from it). If Monzo could install from the Amazon Appstore, then it could install on a vanilla (AOSP) Android (I guess).
Another alternative would be if Monzo ran in a standard Web browser - which would free it from both Apple and Google lock-ins.
Monzo talks about being open source:
https://monzo.com/blog/2016/09/19/building-a-modern-bank-backend/ [https://monzo.com/blog/2016/09/19/building-a-modern-bank-backend/]
but then chooses only closed platforms to run on…
The phone I would like Monzo to run on, but which it doesn’t is here (btw, it’s not a Nokia):
https://puri.sm/products/librem-5/ [https://puri.sm/products/librem-5/]
It’s really all a question of trust and which trust model you believe in - do you trust huge multinationals like Apple, Google and Microsoft to look after you or do you trust individuals able to browse the code and who scream and shout when they see something they don’t like?
At the moment Monzo seems to be looking both ways with a foot in each camp - which is problematic whichever trust model you choose.