> Not all software keeps working for eternity without updates.
True. And yes, Debian stable packages do not work so well if a package needs to undergo frequent updates. As others have pointed out there are work arounds in place - like backports. Or if you want a Ubuntu / Arch like experience, you could try Debian testing. But they are kludges and for some packages the pain got so great Debian was forced to brake it's own rules and pushes new version into stable. But that's rare - Debian stable is so popular because it's stable.
The underlying reason for that focus seems to be Debian is a distribution put together by sysadmin's. A large chunk of the DD's (Debian Developers) are sysadmins pooling their resources to produce a distribution they can use in their day job. The two things that matters to them are security and stability. It's not an accident that Debian led the way on reproducible builds. This mob really cares about rock solid and secure.
To me Debian model of "sysadmin's pooling their resources" is remarkable. It's one of the few open source driver projects that combines open source and commercial objectives. At its heart is a whole pile of people teaming up in an open source project to create a tool (a server OS) they base their respective (and disparate) commercial products. The open nature of Debian - it's ethos, it's transparency, is critical to why this works so well. It allows the participants who might well be producing competing products to trust each other, and continue to work along side it other for decades now to produce a distribution. It also means it's always going to have a strong server focus.
That focus doesn't mean not to say Debian can't be used for a Desktop. A stable base like that is a very attractive thing to build a desktop on, and that's what happened. It's my daily driver, and I think it works wonderfully - but I also hand install some things from outside Debian (like youtube-dlp), and make liberal use of docker containers when I need to pin certain versions for development. It's a bit of work - but as a developer it's stuff I have to do anyway. It created an opportunity to create a derivative that caters to the desktop users who aren't like me and just want the latest shiny - and Ubuntu stepped into it.