Games have no right way to play, stop saying random b/s just because you want to be smart or an elitist. You'd be amazed how many people play games in ways you'd never even dream of. Some people play games to challenge themselves, doing speed runs where they take no damage.
I understand your point, but it's overstated. Of course there are design choices that go into a game, and they shape ways in which a game will be successfully approached.
It's clear that at least one of the values ArenaNet has designed for is approaching the game in a broad way, moving through a variety of content. Now there are also desing choices that run counter to this, like fractals, monthly achievements which require fractal runs, and ascended gear. Which just goes to show that design can have mixture because there are competing goals and values that a designer is trying to meet.
If someone was trying desperately to force elaborate roleplaying experiences into a straight-forward shooter, you could point out that the game wasn't really designed for that, and there might be better options. Of course someone could insist and even manage to make an RP experience happen, but it wasn't part of the design team's intention, and therefore the game doesn't do anything to support it.
Yeah, everyone can play a game however they want, with varying levels of success. That doesn't mean there isn't intent behind game design.
People can complain about diminishing returns, for example, and theorize about the monetization motivations behind it. Or, perhaps it's simply a choice to try and discourage farming of one single area of the game, and instead move through a variety of content so that DR doesn't take it's toll.
Or the long grind to legendaries. Perhaps they're simply a gem shop ploy. Or perhaps they really are something that you are supposed to reach somewhere between six months and one year of play time, just following your bliss through the content. If you are very goal oriented with it, it looks like a ridiculous grind. If it's just something you figure you'll earn enough to shoot for some day while playing the game, perhaps it's not.
I'm not so naive to believe that there aren't monetization goals mixed in here as well, but there are plenty of signs in the games design that there is intent to avoid the farming/maximum efficiency mentality, whether or not it has been successfully achieved.