[UglyToad]

You're exactly right. This is the crux of it. In many domains (not every domain, but every one I've ever worked in) you can delete/rewrite/change the app code without much fuss but the data is vital.

For that reason having constraints enforced by the system that stores the data, external to the app code which developers will inevitably mess up, is so useful and important.

So many issues in my experience have been similar to the one you describe. If the right constraints had been present so much work and so many headaches could have been avoided.

I feel like people who claim they're not needed or not important or "the app code will do it" need to be wrangled into maintenance work of old systems for a year or two until they repent. Rather than boshing out an ill-considered prototype and then moving on before the bugs are discovered.