Bottom line is this: Grading is subjective to the person doing the grading, whether it is us or a third party. A true book's value is not determined by the grader, but by how much someone is willing to lay out for that book.
IMO Mint comics do exist, however they are extremely rare. Just because a book has been handled, does not automatically mean that it is no longer mint, but probably.
I agree that the Printing/Bindery process alone will almost always create some flaw that prevents a comic from reaching this category, and then as you mention, shipping handling, etc., you're lucky if it's still NM.
But, that's not absolute. If you look at the terminology alone, it's not quite right; comics are not from a "Mint."
Coins are. That's where the term is borrowed from. And, what it means is a coin that is in the exact same condition as when it was "minted," (nothing stating that it can never have been held, or shipped) (and of course no production flaws).
Comics are printed and bound, so perhaps a "Mint" comic should really be referred to as "Print," (as in the exact condition as when printed), but again does not mean it could not have been held, or shipped) But here presents an entirely new set of problems with the terminology. We all know how rare it is for a comic not to have some kind Printing/Bindery flaw, no matter how small, that ultimately prevents it from being "Mint."
As far as subjectivity: Yes, certain aspects of grading are subjective. BUT, there are rules too. And there is what is generally accepted as a certain grade between experienced colectors. So, some subjectivity is always going to be there, but too many collectors use it as an excuse not to learn how to properly grade, or overgrade.