When it comes to upgrades and the file system, the folks Apple has are some of the biggest perfectionists I’ve seen, but also some of the most ingenious problem solvers when it comes to making transitions.
I poked around at what Archive & Install did back in the day over a couple major releases, and it was rather interesting. They had built a process for archiving the old system, installing the new one fresh, and then identifying what could be safely deleted and what could be safely imported into the new system (the interesting bit). After a few iterations, this became the default way of upgrading the OS, this identification step became so reliable. And more recently, the migration to sealed & signed system partitions may have nullified the point of such a system, but even that migration is interesting as Archive & Install likely helped inform the design of the migration to the layered FS. Just absolutely impressive stuff that they’ve evolved this, brought along tens of millions (if not more) Mac users to APFS and protected system partitions without people noticing in their day to day.
I used to work on a team that owned engineering processes for an OS-scale project. One of the axioms we had was that “nobody notices when everything is going perfectly, but they will be extremely loud the moment something doesn’t”. Unfortunately, it meant that sometimes we didn’t get budget to address certain scaling issues until it was starting to break, because fixing something and not having the work go noticed was considered a bad thing. I think these folks deserve respect for making such large underlying changes, and having the users be so quiet in response.
But yeah, this sort of testing approach is about the only way to get a real representation of what users are doing and find the edge cases you don’t have internal coverage for. This is one of the reasons companies go all in on telemetry, although the quality of that telemetry varies a lot.