400,000 uniques/month in a consumer play is peanuts. They didn't shut down because they couldn't monetize. They shut down because they didn't get the traction needed for consumer ventures.
We didn't really have many options. We tried to build a business and failed. We could have tried to raise a bridge financing -- but what are we bridging to in that case? A financing (no), an exit (also not an option, as we found)?
Ultimately spinning things down into a skeleton state and doing our best to operate the service for the coming months (heck, maybe years) was the only path forward.
Re: the OP, I think 400k MAUs is actually pretty decent for a mobile app, but maybe I'm mistaken.
We make more than $1 per monthly active user and I know for a fact that our game monetises poorly when compared to the free to play games that monetize very well.