There is a difference between obsolete and out of style. Ultimately there is not much difference in top speed between any of the designs within a formula, the difference in many of the newer designs resides in the small refinement that makes some of them easier to drive at that top speed. Take a newly made (read still tight) MKIV made with a new rig, foils and the current layout and you would be hard pressed to see much if any significant performance gap.
The top guys in any class buy new regularly. It is a lot more fun to have a new design to play with when you do this than just a shinny version of the old – don’t you think?

Anyway you are never going to get rid of the guy who thinks he can just go purchase some more speed cause he can’t make his current equipment win.