Generalized a bit, unit tests on all projects serve to make progress easier and regression harder. There are secondary benefits on evidence/documentation. When the ROI on them turns negative, just delete them. Overcoming the belief that you can't delete unit tests is the lowest cost way to start writing more and better tests.

It's so weird when you think about it. People don't delete unit tests because not having them is bad. So, they... don't write them? "No unit tests" is a state we can always get back to in a seconds. Don't be afraid to leave it.