It's not as simple as you think, the game is designed from ground up with 4 players group in mind. Not so big deal for the trash mobs, but could have big impact on BOSS fights.
I can only hope when more ppl max their first choice job, some of them will level up a tank/healer class so if they find DD queue time is too long, they can just queue as Tank/Healer.
Although I'm not currently planning to level a second class since I hate doing FATE....