is DEX useless now?

Oh, I agree.
I viewed my Cygnus just as you say, as a somewhat more-difficult-than-usual quest for a permanent 12 attack buff for my main.
I'm not complaining about her collecting dust.
I was actually agreeing with you telling the OP that as much as he liked TB, it becomes useless at 120.

My post was not directed at you so much as to the Don Quijote of Cygnus defenders, out battling the windmills again.