^Eh, a fine piece of artwork is valued for it scarcity - there's usually only ONE.
If there was only one birkin and she did this to it - gasp!!! That'd be shocking.
But there are thousands and thousands out there. I was at an event last night and saw at least 6 birkins (and that's in middle America, not Paris or New York). They're not common, but they're not THAT scarce. And you can always get another one. As long as its hers, she can do whatever warms her heart with it. Jane Birkin herself (and her daughter) had a rep for knocking and trampling their new Birkins to make them more worn in and pliable.