Its an idealistic view which will almost certainly fail. Test suites, as much as we like to hope that they reflect real usage, mostly dont. A simple example: if function a gets changed from o(n) to o(n^2) but otherwise behaves identically, most test suites will still pass, but if a user has that function in its own inner loop you can go from o(n^2) to o(n^2^2) which can definitely break a lot of things (simple example: transaction was holding lock for too long and so the transaction was aborted). Being able to catch the above is a high bar for a test suite which I'm fairly confident most test suites are way below that.