They could also hire on a temp basis to get large work loads done. This is a popular thing MMOs do. They'll have a larger initial dev team, then a smaller team to maintain after the major development is done. However, that also requires money and if Muse has enough to afford that kind of setup. Adventure mode wasn't reached as a goal in the kickstarter. It was teased but not reached.
Trouble is, everyone forgets that and part of the reason why is because AM sounds just so damn good. It is the kind of game world many of us are dreaming of and I'm sure Muse dreams of too. In fact as COOP has progressed, it has taken on more ideals of what I was expecting AM to have. Originally I thought COOP would just be a simple COOP vs bosses or objective fights but now we've been teased a SCII Arcade conquest mode where we can join nations and conquer the world. So I wouldn't be surprised if eventually COOP takes on more AM features as it progresses.
