A Phalanx Strength Warrior and a Scepter/Dagger Elementalist both stack might and fury for their party, but to suggest that they necessarily have the same playstyle is pretty erroneous. One class pushes 2 buttons and stacks 12 might for their party spinning and winning against a wall while the other is like playing the piano, rotating between blast finishers in two attunements and half their utility skills.
Now you're absolutely right about roles. A warrior can effectively replace the might stacking role of an elementalist in a party, and vice versa. Guild Wars 2 is a game designed around the idea that any party composition can clear any dungeon/fractal in the game. Some party compositions will inevitably be more efficient at it than others, but at the end of the day you could run FOTM 50 or Arah with five necromancers and do all right.
It has already been done for hilarity and science.
So while elementalists stacking might is great and all, this job can be easily replaced with a guardian, an engineer, a thief, a warrior, or a combination of them. And why not? What's the fun in sitting around pinging the LFG tool trying to get an elementalist, or any class specifically? The sharing of roles is precisely what Guild Wars 2 is designed around, freeing the community from the need to have X class in Y situation. And this rule does not ever get broken, even in the highest end of PvE like Wurm, Tequatl, and FOTM 41-50.
I also kinda have to call bullshit that everything is the same. The speedclear meta is fairly specific in its demands. There are stealth skips that can only be done with a thief. There are projectile wall methods that work only with a mesmer with untraited wardens. There are corner stacking methods that only work with the mass stability from a guardian. While there is considerable overlap between all classes in the game, they're all inevitably "the best" at something. If such weren't the case, then mesmers and guardians wouldn't be so valuable in high-end fractals, and necromancers would be considered equals to warriors.
Whether or not they choose to add a new profession is irrelevant to me. But the suggestion that GW2 must create new roles for new professions is bothersome, because you've so sorely missed the point of GW2's design. It's not about roles. It's about playstyle. And yes, there is a difference.
And yet to think: in GW1 you often straight-up pulled skills from other professions and used them on your bar, including elites. You really think class differentiation is worse in GW2? Many builds I remember running, like E/Mo Ether Renewal, had more skills from my sub-profession than my main one.
And then there was super weird shit like R/N Touch Rangers that just simply don't exist without the help of their sub-professions, which were pretty significant departures from the Ranger's original design and aesthetic. Some of the departures went as far as to not only push sub-profession skills over your main profession's, but to push you toward using weapons from other classes.