Django admin now ships JQuery
code.djangoproject.com
code.djangoproject.com
But the Django admin is only an optional application. Your apps may use whatever makes sense for them
Totally pragmatic.
As for extensive customizations, how much additional concern does this add? It seems unlikely that those people aren't also using admin hacks to work around the areas which didn't used to be user-serviceable so they're probably going to need to be careful about upgrades for many other reasons. This at least gives a consistent baseline and widely-used extension mechanism for future work.