That's kind of how open source works, though. Of course the backend vendors won't care about anything that doesn't affect the backend somehow. But the people, i.e. users, who do want to be able to easily switch away from bad vendors, have incentives to keep things properly decoupled and working.

The license is the key enabler for all of this. The vendors can't be all that sneaky in the code they contribute without much higher risk of being caught. Sure, they will focus on the funnel that brings more data to them, but that leaves others more time to work on the other parts.