I would say the majority of people who take mileage runs only do so around the end of the year when they find themselves a few thousand miles or a few segments short of the next status level. These folks are spending a few hundred dollars, not a few thousand (and they would likely never consider spending that much just for a higher status level as the expense simply wouldn't be worth it to them).
The folks who are spending several thousand a year are probably a much smaller subset. I can't imagine many doing it from "scratch", almost certainly most will already fly a number of flights for business and leisure. Many will often spend a few days if time allows at the destination to make the longer MR's a short leisure trip.
Anything to support your surmises about the "majority" and the "smaller subset"? If the majority only undertook MRs "around the end of the year," then would we expect a marked difference in the number of posts in the last quarter and the first quarter, with many more in the former than in the latter?