You’re complaining about several different things. No one likes matchmaking and the difficulty will reflect that shitty and inherently limited design. But difficulty itself is a different topic and should be based on whatever the devs feel like enforcing on players. If they want players to choose, great. But if they choose only one difficulty for everyone then suck it up or play something else.

A lot of multiplayer games already have casual mode. You’ll still get barely good players getting upset at worse players beneath their barely serviceable skill level because that’s the nature of competitive games. As for co-op, there’s not really matchmaking unless I’m missing something.