Not sure about it being a classic, this decade.
Given the MK5 is the GTi that got its mojo back after staid Mk3s and Mk4s prices will eventually bottom out, but clean examples will become sought after and desirable over MK3s & 4s.
There has always been a trend with golfs and applying the styles of later models - for example lights etc, to older models, and modding in general. This will ensure a variety of cars and prices on the market for a few years yet from unloved, badly modified, professionally modified, standard, well maintained and looked after etc, but as the knackered and crashed cars diminish the numbers, the clean and well kept shouldn't depreciate too much if at all.
My personal opinion is like most versions of the GTi, there will always be enthusiast interest, especially for standard, well maintained or sensibly modified examples, with history, a certain number of doors and probably manual boxes etc.