This is gonna be like an essay but I think it's a really interesting topic.
There are points at which you can get so deep into a fetish that it becomes a lifestyle, and in the case of a lot of fetishes - not just sissification but also sub/dom stuff, pet play, etc. that lifestyle becomes an identity. Your sex life, cause it's so enjoyable, becomes a larger and larger part of your overall life, and because we're molded by our experiences, when your experiences over the course of a few years have included a helluva lot of playing this certain role (in sex play), it just becomes who you are.
As weird as those cases are, I find it noteworthy that identities formed in them are actually still valid when it comes to trans people.
I've said this before about Jessica Fappit. She epitomizes the idea that you can think of yourself as a boy in your young life, but get into feminization, and if you get real reeeeaaalll deep into it your "sissy" self just becomes your self. It's for that reason that I don't think transitioning "because of a fetish" makes you any less trans. While some trans girls may have made the move into that lifestyle because it turned them on, at the point that you make the decision to "come out" your fetish is now just a part of a larger conception of yourself and your identity. And when it comes to gender identity that's pretty much the end of it. Even if it developed from something external rather than the more well-exposed narrative of "i always knew I was a girl," if you feel so strongly that you are a woman, at any point in your life, that at that point you assume and present that identity to your SO, your friends, or to the general public, then you are still a girl.
Anyway yeah, I've heard trans people old and young say they didn't realize they wanted to transition until they saw trans porn and thought "whoa I can DO that?" And I've seen an even LARGER number of trans girls come out of gamer/weeb/NEET internet culture and transition cause they jerk it to a lot of trap hentai, a lot of them obviously stumbling upon it while browsing 4chan underage, and so from their early/middle teens they've wanted to be a cutesy femboi and then in their early 20s they just go for it.