You also have to learn everything that goes with playing a different class. You can sit there and say that because you're a tank you can get a relic done considerably faster which it may seem like however you then need to learn a completely new role, need OTHER armor, and everything else that comes with it. Yes, maybe it would help but it's not something you can do often. In addition to this your example only applies to those people who are grinding these out one after another which the time gate is aimed at stopping. On top of all of that, so what? More tanks, more healers, more money for SE that's not bad for anyone.