Realistically many women don't like it, but some like it a lot. Generally speaking your dating chances are improved by something like that (popular with a subset of girls). Having every girl think you're okay gets you nothing, you need SOME girls to think you're hot.
Long hair is feminine. If you look and project masculine, it gives you a sensual romantic/barbaric wild edge. It's the exact flipside of women with short hair - a look that almost only works on women who are cute and petite and otherwise feminine. If you look like a pirate or barbarian, keep the hair, some girls will LOVE it. if you look like a basement dwelling introvert who is trying to look like some rock singer he likes, cut it.
My husband has braids all the way down to his ass, and he's had some form of long hair for 30+ years.
As for "success in the dating world"... I mean, if that boy has been rejected even once in his damn life, that'd be news to me.
More to the point—Why would you cut yr hair, just for "success in the dating world"? That's just all sorts of bad. Bad idea. You should not tailor yr own appearance, against yr own nature and yr own preferences, simply to attract a larger QUANTITY of other people -- even if that would actually happen, which is doubtful.
Why would you even want to do that, anyway? Would you want to attract someone who's attracted to something that isn't truly you?
I really think it depends on the guy and the girl.
It's like girls with short hair. Some can pull it off, look gorgeous, with it. Many others might really lose a lot of those aesthetics many of us associate with femininity through our social upbringing and are attracted towards.
Some guys look really geeky with long hair. Johnny Depp can probably attract a number of women regardless of hairstyle. He's like the girl who can have whatever hairstyle.
If your the type to work hard and stay passionate then its should all be good , but if its long cuz your the lazy type and its just a byproduct of that then.. well get up, get a job and a hair cut , you gotta earn long hair sex appeal XD