Its the speed. The faster people are able to devour content, the faster they need new content to keep them entertained. New content has to regularly (though not necessarily always) obsolete the older content in some manner, else there would be no reason to do the newer content.
Would anyone be happy if the new HM dungeons dropped i60 gear (same as the current HM dungeons) but are as much a difficulty increase over Pharos as Pharos is over WP? Would anyone really want to bother with WP if the new HM dungeons dropped i70 gear, 100 philo, 50 myth, and 30 newtome, and was equally easy as WP?
If it ain't an upgrade in some manner, people aren't going to bother with it. If it IS an upgrade in some manner, then its gonna be part of obsoleting the older stuff. Plain and simple.
