Although if and when a new Buffy or Angel TV show or movie came to pass, those comics would be "de-canonized" so fast your head would spin--because nobody in their right mind would expect the TV audience to have kept up to speed with the comics.
Again, though, the same is true of "primary" canon -- it can be overwritten too. See Bobby Ewing on
Dallas, or all the other series that just pretend old canon never happened. Stories are works of invention, and that means they are intrinsically capable of reinvention. The biggest misunderstanding of canon is that it's some kind of guarantee that nothing will ever be changed.
...but at the end of the day, only the studio decides what's cannon.
Nobody decides, because canon is just a description of something that happens automatically. The stories told by the creator of the series, or by the creator's officially designated successors, are the canon by
definition, not by declaration.
As Greg said, canon is a non-issue to studios, because what they create is
automatically canon. They absolutely do not devote a moment's thought to "deciding" whether to call something canon, because it doesn't matter to them. Sure, Lucasfilm and others have made assertions about what is canon, but those assertions are for the fans' benefit, and they apply only to tie-in material, since the original work itself simply
is the canon.
"Canon" is a word that
fandom invented (or rather, appropriated from religion) to refer to an original creation (specifically Arthur Conan Doyle's Sherlock Holmes stories) and differentiate it from pastiches and fan fiction created by other people. It's a label for critical analysis and classification. A studio or a series creator doesn't "decide" that their creation is canon any more than a bear decides to be a mammal. The label "mammal" is simply a word that observers invented in order to describe the bear and classify it as distinct from other categories of creature. And the label "canon" is simply a word that fans and critics use to describe an original work from the creators of a franchise and classify it as distinct from other categories of fiction within that franchise (i.e. tie-ins and fan fiction). The only time it even remotely becomes a matter for debate is when the creators deign to extend the label to tie-ins. And any such usage is going to be somewhat artificial.