> instead preserve arbitrary modifications customers have made

Which, having worked in a PC repair shop in a past life, makes it difficult to both diagnose the initial issue and to test the fix. How do I confirm it's a hardware and not a software issue without knowing that the base platform state is in working condition?

Dell machines nowadays have very comprehensive diagnostic utilities baked into the firmware/BIOS, and have had such diagnostic utilities baked in for almost a decade. Booting into any operating system at all, let alone overwriting the user's, is very rarely necessary, and even if the former is necessary, that's what a bootable USB stick or CD-ROM is for.

That said, I agree that there isn't really an expectation of data preservation, which is why removing one's hard drive prior to sending a machine on for servicing is a best practice in most circumstances (the exception being when the hard drive is the faulting module).

On my Dell's, the diagnostics have been a partition on the hard drive, which while in hardware, should not be considered 'baked in' in any important sense. If a user has formatted over that partition when installing another OS, then those diagnostics cease to be available [which was how I came by my intimate knowledge]. Fortunately, the box has never required service beyond BIOS upgrades which can be user installed.

That's interesting, to say the least. I thought they stopped doing that quite a while ago now that UEFI is becoming the norm?

I used to manage a wide deployment of Dell workstations across multiple locations in a small hospital district. We had a Windows 7 image that we'd deploy (nuking all the partitions in the process, including any Dell recovery partitions), yet diagnostics would continue to be available for those times when we encountered hardware issues and needed to send machines back to Dell (and I'm pretty confident that we didn't preserve any diagnostic partition(s) during the imaging process).

I suppose older machines probably required a diagnostic partition, in which case I stand corrected.

You can boot of an external drive, and test it there.

Having worked in internal support at several places, this is the standard practice, since deleting our user's data was the worst that could happen.