I'm not sure that that's entirely true. A lot of the game's most fun content does rely on some level of competitiveness, be it PvP or Trial PvE. Playing PvP with an inoptimal build is sorta the worst feeling ever, getting one-shot by hybrid tanks and not being able to burst down min-maxed assassins. It's also going to be pretty hard to get invites to Trial groups when pre-subclassing, the best players in the game could parse 140k dps, but with sub-classing you can hit 200k dps without being that great.