It's not quite that simple. Ubuntu is a fork of Debian, but not Debian. Trying to install Ubuntu packages on Debian or the other way around will result in a complete mess. They are different and incompatible OS. That they share some roots doesn't really help much.

Debian was never flexible enough of an OS to allow all the customization that Ubuntu needed to do to happen in Debian itself, thus it had to be forked. That to me is a failure, not a success. Forks should always be the absolute last resort, yet are common place with distributions, which creates a terrible situation for the user, has features they might want end up being spread across numerous different distributions.

Cross installs of packages almost always work fine IME.

I used to have a few PPA's enabled as additional sources on my Debian system. It never caused me major headaches.

Debian has a whole notion of a "derived distribution", and is very flexible. I'd love examples of what constraints prevent people from making their own rebrand..

No, Ubuntu is a derivative of Debian. You can easily check for yourself what percentage of packages are taken from Debian without any modification.