HDMI or anything like it was necessary for MCH audio transmission. SPDIF was not capable for that by design. Sure they could have updated it, but the goal was to reduce cable counts and simplify things with video in the same stream if needed. Yes copy protection was a big part of it and they have a designation abbreviation for it too.[ HDCP] however it is not exactly hidden from the public. HDMI specs are moving target since the constant change in need for transmission, mostly of the video side of things. For audio HDMI 1.4 is all one need probably even lower 1.2a. unless you have some crazy 32/384 or larger files to play.
Yes, that's the "Kool-aid" line. We "needed it" for the bandwidth, and we "needed it" for the single-cable solution.
No, we didn't.
At the time of HDMI's inception there were already two viable single-cable solutions that could have been further developed, and actually have been: IEEE-1394 (Firewire), and SDI. Since then, both have progressed too, and now HD-SDI can interface with HDMI and handle 4K and all the audio formats just fine, but with a single 75 ohm coax that is easily field-terminated with BNC connectors, and has far better maximum length capability. IEEE-1394 is a data transmission method, and support multiple chained devices. Neither was considered for the consumer "single cable" solution because neither has built-in handshaking for copy protection. HDCP answered that need for content creators, which was key to having the whole consumer HD video thing work. No studio wanted consumers to be able to get a studio-quality digital copy of their content for free. Of course, that's stupid. It's already a moot point, and easily circumvented. Its Spy VS Spy. The Spy lost.
Since then, we got DisplayPort, a rather thought-out but ill-fated single-cable solution. You know what one of the primary differences between HDMI and DisplayPort is? HDMI comes with a recurring and per-unit licensing fee.
And, of course, DisplayPort was late to the table, late always looses. But don't anyone think that HDMI was a necessary solution. I was an agreement between content owners and hardware manufacturers to block consumer copying, first, foremost, and fundamentally. It's badly engineered, suffers from mechanical issues, electrical issues, length limits, and is not field-terminatable.
Everything else about HDMI was already covered by the existing tech, or could be with very little effort.
Now parallel that with MQA. See what I mean?