Deduce how important a game is to people by introducing small amounts of artificial delay when playing it. This could be input delay, or maybe the game freezes up for a second every minute, or it takes a bit longer to launch or something.

Measure the dropoff rate. People should be really willing to keep playing great games even when inconvenient, and drop marginally-fun grindy games faster. With a large enough install base, you shouldn't need to degrade anyone's experience very much to get enough data.