Now, it's completely true that if one stretch goal is about cooperative multiplayer, it means it won't be something else.
So, if you're against that - it means you don't want other people to have what they enjoy - even if you still get the full game, as well as other stretch goals oriented around singleplayer features.
That's the case for everything you choose to implement, however, and for whatever reason - multiplayer is much worse than X singleplayer feature that isn't necessarily what YOU want.
I'm not saying it's the case for you - but a lot of people seem to think that multiplayer, specifically, is ruining games. This strikes me as pretty irrational and a bit selfish, but that's ok. People are people
The thing about cooperative multiplayer is that it's relatively easy to implement in an engine like Unity - because the network structure is mostly in place. Sure, it will take some work - but it's not likely to ruin anything at all.