maybe there is an actual explanation for HDMI Forum’s decision that I am missing.

HDMI has never been an open standard (to the best of my understanding anyway). You’ve always needed to be an adopter or a member of HDMI forum to get the latest (or future) specs. So it’s not like they’ve just rejected a new idea. The rejection is fully consistent with their entire history of keeping the latest versions on lockdown.

Standards organizations like HDMI Forum look like a monolith from the outside (like “they should explain their thinking here”) but really they are loosely coupled amalgamations of hundreds of companies, all of whom are working hard to make sure that (a) their patents are (and remain) essential, and that (b) nothing mandatory in a new version of the standard threatens their business. Think of it more like the UN General Assembly than a unified group of participants. Their likely isn’t a unified thinking other than that many Forum members are also participants in the patent licensing pool, so giving away something for which they collect royalties is just not a normal thought. Like… they’re not gonna give something away without getting something in return.

I was a member of HDMI Forum for a brief while. Standards bodies like tihs are a bit of a weird world where motivations are often quite opaque.