That's a very unusual problem, I'll be honest I always have work.

My two cents,

If you have a solid team that has the ability to build a successful product do not loose them. Finding a team that can execute quickly and successfully will get you a long way and if you have one. DO NOT LOOSE THEM.

But I have to say, if the product is successful then in theory it should be growing like mad and that should pose multiple challenges on the infrastructure and the product itself. The team will have enough to do to keep the lights on else the product itself is not successful and needs something more to make it successful.

Finally, if the product is actually successful and is not growing due to external factors then

1. I would build a new/tweaked product that the market would accept
2. If you are confident about the product, I have put developers on support calls, user demos and shadowing sessions so that they experience what the users / customers are experiencing.
3. I like to have them as a fly on the wall in prospect conversations to give them context of what the market is saying ans what the needs are.
4. IF you have truly done all of the above and have a successful product, I would send the guys and gals on vacation and tell them to enjoy. They will be doubly productive when they come back and more appreciative than creating random unnecessary work for them.

But again DO NOT LOOSE THEM IF THE TEAM IS SO STELLER. (Actually can I hire them?)