On the subject of game development at Valve, there's a lot of things that have been worked on but never turned into an actual release. There were 5 cancelled Half-Lifes between Episode 2 and Alyx (Including Half Life 3), Left 4 Dead 3, F-Stop, "RPG". A.R.T.I, SimTrek and probably a few more.

That sometimes over-cancelling willingness I think is also part of what makes the end result great-TF2 took 9 years and several versions similarly to Alyx, but I think both were worth the wait.