Linux Mint, Pop, and more, all now provide "Windows without MS" which is what most users really want/need. Even with gaming now that Steam has multiple ways to run on Linux. You can put grandpa in front of some Linux boxes today and he won't notice the difference.
Debian, BSD, etc. I totally get you want the most stable thing going. You like to pretend that memorizing arcane spells for 20 years is the same as intuitive. I get it. I run some remote stuff on a BSD box. I don't know a single serious command without a lookup. But it's stable. I thank the guy who made a cheat sheet for common commands.
Even on the security front though, the tinfoils are doing weird containerized OSes and stuff now. Opening a browser tab can spawn a whole sandboxed OS in seconds. Your pitifully out of date Debian repo doesn't really stack up anymore.
Basically. You can run a modern OS that doesn't suck to use. And that OS can be Linux. With Windows 10+ poised to drive many away from MS, distros need to get with the times and be ready for the masses.
Or. "Serious users will move to X" is a bad argument from the 1990s. Newbies have an OS that pros can enjoy. And if I want to run Debian, I can fire it up in a sandbox or whatever. It's the 21st century. It's not like I have to wait for physical media to spin up to install a live copy.