It's not necessarily a danger to not release a sequel immediately if they've got other things to work on. People will still buy Half-Life 3 in a few years time - similarly Blizzard have had ten-year-plus gaps between Starcraft - Starcraft 2 and Diablo 2 - Diablo 3, which hasn't seemed to do them any harm.

That's distinct from not working on something simply because it's "boring" or "too hard".