It's the same thing in every trinity based mmo I played regardless of party size and healer/tank classes to dps classes ratio.

It's because most players don't want to tank or heal. That's the problem. It's not the lack of choice, party slots or class design. The problem is the players. There's no cure for this. Even in table top rpgs this sort of thing happens - most people are drawn to classes that specialise in dealing damage.