When you read it as 'a massive pile of legacy Windows applications', which is reasonable, it's not a stretch to assume that many applications written in the last 20 years assume that the OS is on a harddisk, and on C: for that matter.
Hand in hand with 32bit Windows came the registry, which is where most applications stored configuration information until relatively recent times.
To the best of my recollection, I have not seen an application or utility hard coded to C: since Windows 95. But I would be interested to learn of the actual examples which support your assumptions.
I used to have Windows on C:, Program Files on D: and My Documents on E:. Everything broke, to an approximation.