Valve has slowly transitioned from a game developer to a platform developer. They make more money with Steam than they would ever do with a single game, and with Steam they have done the entire gaming community a huge service.

I have always expected Valve to come up with Half Life 3 to boost one of their new toys. For example, release it for Steam Box with VR support and something new/cool in the gameplay; having the portal gun in a more open world game would be awesome...

Never releasing Half-Life 3 won't be because Valve is lazy, if they cancel it I think it would be because the current generation of gamers haven't even played Half Life 2, so the market for it might have disappeared.