That's for the people who refuse to update to the mid 201x's. Then you said what about 4K so if they're updating to 4K they need to at least update their AVR/Soundbar too. No sense in keeping their gimped equipment that they shouldn't have bought in the first place. People who cobble together crappy equipment will eventually have to face reality and either spend $15-$25 for a solution to limp on by or power-up and replace it with something that isn't already obsoleted.
sound systems have ridiculously long life span. It doesn't matter whether you have crappy or audiophile-grade surround system, it usually will still sounds as good despite the age. Even if its broken, it usually still can be fixed again and again and again. So there's no reason to upgrade sound system to the new shiny stuff. Even less reason to upgrade if your old sound system is audiophile-grade or THX certified and calibrated, etc and its already sounds godly.
looking at ars comments, it seems even people with expensive TV like LG OLED TV still use surround sound system with optical audio. They probabaly have audiophile-grade surround sound system, as they able to afford expensive stuff like LG OLED TV.
then, if you do upgrade from 5.1ch to atmos with height speakers, it could be a real pain to setup depending on the design of the structure of your home (is ceiling easily accessible?), the interior design, etc. Should be relatively simple if you upgrade from 5.1ch to 5.1ch tho, as nowadays there are wireless rear speakers. So you can just plops them in the old locations. You probably just need to do sound calibration, nothing else.
But then if you already have 5.1ch, why upgrade to 5.1ch that could potentially sound just the same as your old sets that works just fine, or... sounds even worse. So go straight to atmos.
I'm sure MS already have the audio usage statistics.
So its probably that most people only use 2ch, and those that use surround sound systems probably are people with new sets with HDMI audio, or rich enough (they probably can tie console audio device usage to content ownership or spending on ms store) to have the money to throw away (or sell or gift) their old system and buy new ones and hire professional installer and audio calibrator.
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Now what i'm not sure is, does this means Xbox Series X will only able to ouputs LPCM and Atmos over HDMI? Then not only those with historical stone-age sets are screwed, but those with HDMI input (but no atmos support) could be too. Because HDMI LPCM has been limited to stereo for years. Only sets in 2020 or end of 2019 have 7.1ch over HDMI LPCM AFAIK.