Its really easy to make a custom rom but hard to do serious "real life" stuff; companies don't want to make it easy. To most regular users, if they cant download apps from the google play store, and they can't use venmo\cashapp, then the OS is dead in the water from day 1
Trying to get some scale, you're hypothesizing about giving 10 millions to HSBC to make business with your startup, when they're throwing away 500+ millions every year just to cover their money laundering.
https://www.investopedia.com/stock-analysis/2013/investing-n...
And we're discussing doing this for basically every major banks.
I see it akin to the proverbial "not getting out of bed for less than XXXXX". You're getting out of bed every day, for free. But having someone make you do it for a specific reason will be an exponentially harder proposition.
> 1 line in their app
Aren't you asking them to maintain compatibility outside of Play Services and be on available on your platform ? That's a whole project, including their (or their contracting shop's) validating the whole new stack from a security and technical perspective, and a legal and business check on what that actually means to them.
Perhaps we can look at it from a darker perspective: if a random guy came to the bank to ask them support for their parralel phone ecosystem, the bank would at least want to know what they're getting into and what's in it for them. Especially if they're offered 10 millions for allegedly one line of code.