The only way they could justify outdoor players getting Mythic or even Heroic gear is if outdoor bosses became similar in difficulty to the ones from Guild Wars 2 (which are not that difficult but also are far from being loot pinatas. You’ll fail more than you succeed if people don’t pay attention).
GW2 does have what I consider a good solution though which is that there are different gear sets for different activities. Raiders go for gear that maximizes damage but assumes you have a healer present. Open world players go for gear that allow good damage over time but also provides defense. Neither group needs the other’s gear.
Funny enough WoW did this in Legion with the Class Order Hall set but in typical Blizz fashion by the time you grinded enough rep with the Order Hall to buy the open world set you were long past needing it.
Of course for Open World players to get their own gear sets the Open World would need at least SOME challenge to it.