half life releases were tied to new platforms, such as HL2 and its physics engine, or HL Alex and VR kits.

it's like Nintendo having a Mario game for their new hardware, e.g. Mario 64, etc.

there weren't that many teases, nor is it great marketing; CS:GO competitive e-sports is better marketed and probably made Valve more money than any HL wink-wink-nudge-nudge ever would.