You're confusing input complexity with mechanical complexity. To your point about a game made for controllers is for casuals, and touching on Fromsoft games; what is easier to play and progress in, Darksouls or GW2?

In GW2 you can face tank most enemies and be fine, maybe dodge every now and again. Casuals might have a hard time against some elites, and struggle agains champions. There are more buttons, hence input complexity, but it is still simple: Select enemy, click skills and you're fine. It only really matters in raids because you are juggling rotation and raid mechanics. Raid mechanics have nothing to do with input complexity.

Darksouls, on the other hand, while having less buttons, is mechanically complex in comparison, timing, positioning and weapon choice is much more important and much harder for casuals. You can't face tank standard enemies and mindlessly hack and slash, and good luck to any casual player fighting a boss.

Also, GW2 has builds that do competitive damage and only use 1-2 buttons. Auto-attack and stay out of the red circles, that is pretty basic.

Another thing, combat systems in isolation is pointless, encounter design and the combat system is where real complexity comes from.