Yes, there could be a comprehensive list that would cover most reasons.
No, it would serve no purpose other than to refer people to it , to get them to shut-up about why persons are running an older system ;-) that being thier problem, not ours.
"Why do you want to" (run an older system), Asking for a poll, bad.
While the format is no place for a poll, the poll itself would prove to be of limited value because not enough people would be polled, and not enough "types" of people would even see or participate in a poll at SU.
There is no arrangment in the SE software to create an actual poll like many forums would have, and voting on 1 or more comprehensive list(s) would be of no value.
Therefore, I see nothing wrong with creating a comprehensive list of the reasons why people, would do, have done, and will continue to do such things. (but) I see no purpose to be gained in creating it , maintaining it, having it be searchable or found on the web. It may very well instead become a distraction filled with "keywords" that pop-up on search but provide no information for what a person was searching for. AKA not help.
There are expert people who would object to someones opinion of why they would be using an older OS or even hardware, and feel the need to correct them, edit thier post, or add commenting, because of that. Some of that could be valuable, but the discussions could become endless, and be argumentitive opinion, instead of methodological assistance and fact.
There are also experts who know exactally why and how to use an old OS or Old hardware, That type of information would be very valuable.
like: How do I secure a specific port on an XP system that we're stuck with that runs some machinery. How do I keep this old OS from ever going on the web at all ,etc. (tons of other specific questions)
Information of that type IMO would be very usable, if it didnt lead to #1 argumentitive opinion instead of assistance and fact. Questions of that type have been done before and work ok.