Some men are energized from being around what they desire, it gives them the strength they need to get through and make a good impression, if they are anything like me then being around what they highly desire gives them the vapors and their chances of making a good impression are very bad. But I know it's possible to be empowered by adversity like that because I've felt it, very rarely.
If I had to ballpark it, now, in 2018, I would say it's 20% at the highest, simply because men appear to have less ability to choose than ever before, see all the couples where the woman seemed to do all the picking - a short ugly or boring urchin shopping herself a discounted tall GL outgoing male, oneitis doesn't even factor into it.
Chad doesn't have oneitis.
Normie either can't get his oneitis after giving it a good try or prefers to get a fat gf quick and start sharing "thicc" memes online.
Incel collapses completely in the face of his oneitis from the pressure of trying to shoulder both total lack of experience and overexcitement at once.