If I understand correctly, Debian decides to deliver a certain version of a package (say v1.2.3) for a certain Debian version and they generally keep that versi

If I understand correctly, Debian decides to deliver a certain version of a package (say v1.2.3) for a certain Debian version and they generally keep that version of the package fixed (or try to stay 100% compatible with it), minus security/major impact bugs which get backported. By doing this, Debian can ensure that when you upgrade the system, nothing breaks. While it's not uncommon for upstreams to offer a stable or LTS channel that mostly works like this (and generally stable distributions decide to package this version), the whole value of Debian is to offer a layer on top of many upstreams with different speeds / practices / release policies / etc. and offer you a system that works well together and doesn't break. So the work/backports they need to do is mostly related to different upstreams working differently or in a way that doesn't allow Debian to stay pinned on a certain version.