Android shouldn't be considered Open Source anymore, since source code is published in batches and only part of the system is open, with more and more apps going behind the Google ecosystem itself.
Maybe it's time for a third large phone OS, whether it comes from China getting fed up with the US and Google's shenanigans (Huawei has HarmonyOS but it's not open) or some "GNU/Linux" touch version that has a serious ecosystem. Especially when more and more apps and services are "mobile-first" or "mobile-only" like banking.
That is a very hard problem, unless someone with serious name recognition like Linus Torvalds starts to lead that kind of effort, or a big company like Microsoft suddenly decides that putting 1 billion towards GNU/Linux would be in their interest. With small efforts, it will remain scattered.
Crowdfunding has a lot of power if there is name recognition behind the effort. Star Citizen has already gathered $800 million with mostly enthusiasm and a good start. Who is there to lead the effort for GNU/Linux phone development?
But I think Sailfish OS has a mature ecosystem, they are well recognized in the EU and based on GNU/Linux. I use it daily, after moving from UBports, and it serves me well. Hopefully SfOS gains more popularity.
For the new ecosystem to win, it needs to have its own user base for companies building apps to recognize it. Even with SailfishOS, the banking apps still require Android compatibility layer, which is slowly eroded with Play Services and Play integrity check disabling those one by one in the coming years.
I would say that this is really not the OS's problem, but the bank's problem. I find it absolutely intolerable that there are banks that force me to use a OS from one (or two) specific vendors.
Same goes for public transportation services (German Bahn Card is now only available in their app) or post mail services (German Post "Mobile Stamp" is only available in their official app).
Technically not as long as the fallback PDF version remains available.