I don't know if Valve was waiting for the next technical wow factor, I doubt there's anything in entertainment technology that could surprise us, or any other audience of any other medium, not like HL2's physics and lighting did, not even home holographic technology nor sexbots.
The only thing that can still surprise is a good story, and that early draft of Epistle 3 sounded fantastic, they should've released it if HL3 was up in the air anyway, even after episode 3, but I don't have any info to make any sort of judgment on Valve's reasoning, nor does anyone else, this isn't a Kojima situation where we have enough info to make a call, there are barely any scraps of random data to go by.
As is, there's no reason to believe they won't release any more HL in the future, since apparently last scrap of evidence that they've been developing for HL related stuff was 2014, they may very well still be at it, and evidence for L4D3 and portal as late as 2016 IIRC.
I haven't played every single game since HL2 came out, but I highly, *highly* doubt there's a single game out there that has surpassed its story, the concepts in HL and HL2 are too well understood, too well executed and the implications and magnitude too far reaching without being gratuitous in the slightest, such execution is only found in raw sci-fi novels, the story of HL2 is nothing short of a 2001: A Space Odyssey in videogame form, many games have derived from it, but I haven't seen one that has come close.
If Death Stranding is anything like I imagine it to be, then maybe that one has a chance, but I think it's more likely that it will be what Interstellar is to 2001.