DR... Can it disappear Please?
DR is valid as a mechanic which encourages “diversity of gameplay.” You can see this even in CoF farming, where some groups will advertise two or three runs of Path 1, then a run of Path 2 (to reset the Path 1 DR), and then repeat.
It would be very difficult to create a game in which all activities are perfectly balanced in terms of rewards-per-hour. In the worst case, players will discover the single activity which is most rewarding and then farm/grind it endlessly. While doing so, they won’t actually have much fun or build a healthy game community.
In a perfect world, DR would automatically compensate for this problem. The “most rewarding” activity will still exist, but after running it for a half-hour it will become sub-optimal. Truly dedicated/addicted farmers will setup a sequence of activities (e.g. Penitent-Shelter) which incorporates a bit of diversity; more casual players will probably farm until they hit DR and then do something more interesting (e.g. megaboss events, guild missions, WvW, daily achievements, Super Adventure Box, etc). In this way, DR can promote more balanced gameplay.
Also – grinding for legendaries (and/or vanity items) isn’t the only thing to do at level 80.