Well, I understand both sides of it.
As someone with extremely long hair, I understand that having long hair can be a huge pain in the ass; it's harder to maintain, it can hurt your neck, you can't try doing all kinds of crazy new designs because there aren't a whole lot of at home styles you can do with long hair, and sometimes you just want a change.
On the other hand, if you're asking her not to, and you have a fetish for long hair, then she should take that into consideration. I wouldn't want to destroy something that brings my partner a lot of joy.
On the other hand, it's my body and my hair, and I dress as I do and wear my hair (and everything else) the way I want to because it makes me feel attractive.
Have you asked her to consider cutting it shorter but not short? You said that she knows you have a fetish, but also say that you won't tell her not to cut it... If she knows, then why aren't you explaining honestly why you don't want her to cut it?