It has nothing to do with the willingness of others to do a complete reinstall, but the total restriction for those that don't want to do it. So for those that want to do a complete reinstall they already have that option, whilst those that don't want to go down that route don't have that option.
A long time ago when I upgraded from win98SE to XP Pro, that was a huge jump in technology terms, perhaps as comparable as XP to Windows7, yet Microsoft didn't apply this same logic in forcing users to do a complete system reinstall.
This isn't a problem if you are going to switch to W7 only when you purchase a new system, but for those with a perfectly functional and relatively new system, it is hardly going to encourage them to upgrade when that path is made difficult for them. So both the user and MS lose out as a sale isn't made and people stick with what they have.