I think the problem with HL3 is how hard it is to do something truly innovative and original these days and everyone would be disappointed with a generic sequel.
I don't think that would be a problem if they actually wanted to make HL3. They already confirmed that HL and Portal are in the same universe, so if they give us a HL game where you can have both the portal gun and gravity gun that would be enough innovation for me.
HL2 and its episodes still have better physics and facial animation than most games released today, so imagine what they could do in HL3 (simultaneous motion capture on both face and body for all characters - has that been done in a game yet?). Also if HL3 was announced as a VR game that would be all the incentive I need to invest in VR gear.
I think if they get to 10 years since EP2 and still have no plans to develop the series further, they should sell the intellectual property and let another developer give the fans what they want, although I could see that going horribly wrong if it's given to the wrong developer.