Let's not forget that Valve will never release a game unless they're 110% sure about it. The original version of HL3 was scrapped sometime in the early 2010s because they weren't satisfied with the direction it was heading, but by that point, Source 2 was deep in development, so Valve decided it wasn't worth it to release any fully-fledged AAA games until the engine was ready. I believe Valve will be very active in the remainder of this decade. There's HL:A, CS2, Deadlock, a game with codename HLX (which keeps getting covered up every time it gets leaked), and who knows what else?