That requires orders of magnitude more capital/work than what got the app to where it is. At that scale whatever this app already has built is irrelevant.
Not to say this app isn’t valuable, because it is, but that you don’t just build your own payment processor.
To extend the analogy further, that's like saying "I'll just build my own electricity company" when they cut your power.
Thought about this so many times with domain registrars. I’m content not being a huge company. Way simpler.