Paaaaaass. I play 99.5% solo… but co-op is undeniably more fun, and potentially doubles the enjoyment of a game. Or quadruples it (depends). Also, “everything to progress” = “everything”. If you just meant “between difficulty tiers”… paaaaaassssssssss. Extremely un-Diablo-like.
If this were a solid game (it’s not yet), co-op is what would give it legendary potential. As it stands, co-op is basically what keeps a lot of players (including some of the most infamous white knights on the forum) from experiencing just how dull the solo version of this game is.
And since you didn’t complain about trade or (imaginary) leaderboards, you’re only wishing more boredom on others with no upside for yourself? Man, what did life do to you? 