I agree. The problem is that almost inevitably they end up getting more and more expensive to build and maintain while at the same time becoming buggy ;) It's a difficult battle to win.

I think you need self discipline to keep limiting yourself to an ever evolving subset that is the optimal ROI. This means over time removing tests that don't add as much value any more. Rewriting some other tests. etc. Human nature though is that these keep growing endlessly and become hard to manage just like any other part of software.