Progression is a part of every MMORPG that I can recall. Call it raiding, questing, eventing, dailies, weeklies, arenas, RvR, etc, etc, etc... That progression is still there. They are all "grinding" per se. Besides, what viable audience? They've "appealed" to a couple million folks for this buy-to-play game already. GW2's revenue model isn't built around making people pay-to-play and, therefore, needing to keep them wanting to play. They've already got your money, so to speak. So everything Arena Net gets afterward is pure bonus.
Since GW2's revenue model isn't built around subscribers spending days, weeks, months, or even years, grinding raids for progression. Why should their progression have to carbon copy game's whose revenue model is? It doesn't and, it just so happens, isn't.