I think good unit test coverage is essential for good maintenance, but you can't cover _everything_ with unit tests. You definitely need integration and e2e tests to ensure bigger picture correctness. I have found that value one gets out of the different kinds of tests as you proceed up the pyramid get higher the closer to release you are, and are most valuable post release. During development they become obsolete too quickly due to changes in how things work. But locking in an already-released version to avoid regressions is great.
I think good unit test coverage is essential for good maintenance, but you can't cover _everything_ with unit tests. You definitely need integration and e2e tests to ensure bigger picture correctness. I have found that value one gets out of the different kinds of tests as you proceed up the pyramid get higher the closer to release you are, and are most valuable post release. During development they become obsolete too quickly due to changes in how things work. But locking in an already-released version to avoid regressions is great.