I'm against co-op because of my experience in LoTR LCG.
In LoTR, there is 3 missions. After 2 week i build a deck that overkill the first two (it's impossible to win constantly against the third alone) in 95%.
I wait more than a month to have a new challenge, and 2 day after the first pack, I have the same issue that before... So I waiting the next pack...
Cool, I play 2 or 3 time a month.
In a 1V1 game, if I always win, my opponent would adapt and I can lose after. This is why the meta evolve.
In a Co-op, when you find the way to win, you stop playing...