That last line is true to a point, but not in my case and many others. People have been talking about this for months on the beta including myself on forums. The bags not being movable is one example of them not listening to people on the beta. They still have time though.There's plenty of people who raid in super competitive guilds that use default UI. There's also way more arena players that use default UI than most people would like to admit.
I've used default UI for years and it serves me perfectly fine as a raid leader. All I did was resize the UI and everything appeared fine for me. The default raid frames for a tank/DPS is fine as it still shows deaths and necessary debuffs, although I would admit if I was a healer I might seek better alternatives.
Boss mods are important and depending on the encounter weak auras can be important. I haven't raided since the end of BFA, but the last encounter I thought was all but necessary to have weak auras installed was Mythic Azshara. Details and damage meters can be important, but you can also decide to track that shit after an encounter or after a dungeon too. It's a useful tool to analyze, but analyze in the middle of battle it could be seen as wasted real estate.
If you're use to something it's just never going to replace what you had before. As someone who just logged into retail just the other day in months, the new UI is by and large far better than it was before. Did it eliminate any addons for me? No. But people underestimate how many addons it potentially might have erased for others who do use the default UI.
If you sit there and use ElvUI and take hours upon hours to setup things perfectly, I highly doubt you're going to use default UI in any universe. But plenty of people do use the default UI. Popular to contrary belief most people would play just fine downloading simply a boss/dungeon mod pack for raiding/dungeons. I can't deny how good weak auras can be, but flashing icons and audio alerts do nothing if you don't pay attention to them. We have traffic lights telling you when to walk or wait for traffic, but people still get run over despite those flashing lights telling them it's not safe to move lol.
Also a lot of the whining is because people haven't even taken the time to dig into the interface options to see what can be done with it.