Adapters can be used, of course, however adapters are yet another cable, more cost and waste, if you will, and often lag behind new standards, in reliability and signal integrity. Conversion of signal in adapters will lose you VRR, for example, as many adapters are not designed to convert and pass through entire package of features from original signal. It's a pain. One adapter will work with Gsync, another one will not. HDMI 2.1 adapters have not seen any success so far, to the best of my knowledge. If adapters are to be used, those need to be designed properly, without skimping on features. I have not seen any DP to HDMI 2.1 adapter that would successfully handle FRL signalling and package of video features going through a pipeline.PC and laptops have less and less displayport and more and more either HDMI or just USB-C. A displayport to HDMI adapter costs a few dozens of dollars, same for USB-C to HDMI, this is not a big issue.
Information about DisplayPort is not accurate. DisplayPort is the primary video pipeline in all CPUs and GPUs and there will never be less of it; quite opposite. Every single modern PC and laptop have DisplayPort signalling in one of four forms - traditional DP port, DisplayPort signal tunneled either through USB-C or through USB-C Thunderbolt and eDP - embedded DisplayPort for flat panels on laptops or other mobile devices. I have all of them at home, which gives flexibility to connect to literally all monitors. DP is here to stay, not to go anywhere. The fact that it can run over several interfaces is brilliant. HDMI ports also feature on many motherboards and GPUs, which is great, as several interfaces simply give us more choice to connect to whatever display devices we wish, including TVs and projectors. It's about versatility. Just because PC has HDMI port as another option alongside DP, it does not mean that AVRs must stay so conservative regarding interfaces and not support anything else apart from HDMI.
AVR companies could really try harder to finally catch-up with modern, diverse connectivity of home media entertainment devices and start installing at least one or two in/out USB-C with tunneled DP (40 Gbps), to give people more choices to connect whatever they please. It's ludicrous to stay HDMI-only device in 2021 and claim "it's a hub". Silly and short-sighted.
I agree with you that trash is bad enough, hence proposition to well integrate everything in one box, without need for splitters, adapters, transmistters, etc. AR is a good solution too, I support it, and there are plenty of DAC-s that can do audio only.This just protect you from having to trash your AVR in the bin when a new HDMI standard will come. The world would had significantly less trash if instead of AVRs (ie managing audio+video), we just had multi-channels processor/amps, ie just managing audio, ie "AR".
AVRs are not going to disappear. They need to become much better by being modernised. Companies need to change the way they design AVRs' functionality, enrich I/O ports and speed-up systems. They got audio part mostly right and it is pretty mature. Video support and interfaces are lagging behind. Many mainstream AVRs are by decision and design "trashable" after 5-6 years, so that business can continue. Many of AVRs have usb 2.0 only, which is totally bizarre... This does not have to be the case and Trinnov and Mad VR are proof of that - one box that does it all for AV for more than a decade, with free software upgrades and exchangeable parts; just like PC. Modularity is the key. This modularity mind-set is also arriving to laptop sector, little by little. AV processors are modular enough, but still very high-end, niche and expensive. This will need to change if we are to produce less e-trash. As soon as there are consumers willing to buy trashable devices, companies will sell them. So, education and eco-mind-set need to come on both sides. Consumers should expect from companies integrated and modular AV features without need to buy extra devices for conversion, correction and/or processing. The only thing you would upgrade after 5-8 years are individual parts, such as board, capacitors, graphics card, CPU, DAC chip, case or similar. That's why Sound United should have recalled all new AVRs to change HDMI 2.1 chip rather than issuing that horrible adapter box that is meant to correct the signal from only one source on only one port. Ridiculous. As soon as they think "what is cheaper", those are kinds of solutions we would see on markets, no matter how embarrassing it looks for AVR sector.