My understanding is this was the intended ending for just Half-Life 2 (episode 3), with no intention of there being a third game.
The ending seems similar to Half-Life 1. Gordon Freeman is transported into the future again, to be called on when needed, or not. In a sense it’s not a cliffhanger to Freeman’s story, as this is wrapped up in the final paragraph, but it leaves a larger universe that may or may not be explored. The G-Man and Alex may still make an appearance in the future Valve game. Laidlaw revealed on Twitter that he did not know the relationship between G-Man and Alex. So maybe we’ll see them in another game like Portal set in the same universe.
But yes, it is disappointing that this is the way we were presented with an ending to Half-Life 2, only one of the greatest games of all time.