They're contractually obligated to exist until 31 December.
I don't think that's true at all.
I haven't seen the contract but yes, it probably doesn't require them to do stuff; and if it does, at most it would be, if they fail to do stuff the contract can be terminated. And that could even have a 6 month notice or something.
However I also think it's unlikely they will shut down early. For most of the time they'll be up, they have the opportunity to make money. That's good for them so they will go with that. If income slows down to a trickle late in the year, they still have the incentive of wanting to look good to third parties, at the reduced cost of just however much time is left.
Now, odds are they won't keep adding features post-Adventures.