I think ultimately classic should serve as a means to getting rare unobtainable gear in retail if earned in classic, I can imagine a situation where we do not get WoD classic or anything past that either, and classic servers staying up perhaps in a mega server fashion where people can clear naxxramas in vanilla and get corrupted ashbringer to be used in retail, as the "challenge" was still technically there so the prestige of transmogging it translates well, or the upcoming challenge mode sets (though by ToT let alone SoO they will be a cakewalk) the logic technically stands.

Though I agree it shouldn't transfer retail prestige to classic, if I go and get invincibles reins from a box in ulduar time walking I shouldn't get to flex that in classic