is DEX useless now?

Unfortunately, Big Bang was the final nail in the coffin of the Knights of Cygnus.
Before that, a level 120 KoC could participate in at least some "end game" content (such as Maple has). Now, they can't even hit the more important bosses.

Endgame bosses Cygnus can't do:
Chaos Zakum, Horntail's, Pink Bean, Shietty-exp Showa bosses, Time Temple mini-bosses.

Cygnus can do:
Zakum (lvl110), Papulatus (lvl125), Pianus (lvl110), CwK PQ (lvl120), Dragon Rider PQ (lvl120), Bigfoot, Headless Horseman, All Party Quests, All mini bosses.

So, unless you really like the zOMG Horntail / Pinkbean drama, Cygnus are still quite ok...

P.S. About DEX-reset... Don't you need DEX to equip your knuckles? o.0