This gives us some unique abilities in terms of modeling our productivity of course, because we started measuring before anyone thought up unit testing.
Over the past 15 years, unit testing has failed to produce anything positive, and test driven development has been an absolute disaster.
That being said, this isn't something which will be universally true. A lot of the software we use isn't build by us, and I'm certainly that a lot of the suppliers on that software use unit tests quite extensively.
For the things we build ourselves, however, there has been almost no value in adopting modern test philosophies.
You say that systems have to be able to be worked on 5 years from now, but the truth is that most of our systems transport data and rarely live 5 years without getting rewritten to deliver better performance, higher levels of security or simply because the business has changed completely. A lot of it hold very few responsibilities as well, making it extremely obvious what to fix when a service breaks.
Don't get me wrong, we've seen problems we wouldn't have with 100% coverage. But that doesn't matter when spending resources fixing them is still a net positive on every account.