> From me a clear "yes" unless there is something like a coop mission folder which sounds like a bad idea.

But that means either
a) there would be an upper limit in difficulty for those missions that need you to do things quickly
or
b) missions would adjust their difficulty on player count

I understand that not getting all gold stars is annoying. But both proposals to solve it have some weakness. Besides we want to promote multiplayer (because it's just much more fun to play together!). We don't want to penalize it.

Another solution would be to add an extra difficulty level to all missions (something like "Apocalypse") that can explicitly only be started with at least two players and gives you a different achievement if you finish it. So you can earn gold on all missions, but you can only get the double gold star by teaming up. Scaling up the difficulty shouldn't be that hard. We could e.g. add some of the new animals (like Pukas) so one player may have to focus on defending against enemy swarms.