There was not a moment which I didn't expect a sequel to Halo 3.

If a series makes success, the company will keep makiing games of it. And if an once succesful series stops selling good, they will pass it to a not-so-good team to butcher it and sell the pieces.

The best thing which can happen for a game (but not for a company) is to be very good, but not sell very much. If they do the appeal to make it more and more "popular" and "accesible" is just too big.

This, or be made by Valve. They never release a 3rd game anyway.