Now that I've translated your French. To begin with you miss my point and that is that Denon , Marantz,etc 2020 receivers will indeed pass video from games input from PS5 & Nvidia RTX30XX series graphics cards at either 4k@120fps or 8k@30fps, 8k@30fps only requires 24.48Gbps, Denon & Marantz support up to 40Gbps see video, so Denon, etc are not lying at all.
The Xbox uses DSC coding to output it's 8k supposedly at 60fps, this requires more than 48Gbs so Xbox must use Data Stream Compression to achieve this. It also occurred to me that Xbox series X must use DSC processing for 4K@120fps(Hence the Blank screen?) which seems entirely pointless when you look at the HDMI 2.1 specs. Your arguement is a really moot point as there is zero 8K content of any kind at this time and unlikely for the next few years. There are no games at all that are coded in 8k let alone 60 or 120fps. By the time there are your Xbox series x will no doubt have been supersceeded by yet another more powerful console.
They've gotta keep the money wheel spining mate.
Nothing at all wrong with your amp you're missing out on far more important things than 8k such as Dolby ATMOS & DTS:X also when the current gen 4K TV's catch up you'll have VRR, ALLM, QMS & QFT all of which your Denon amp supports.