I think you need to be one (in terms of value) in order to keep one for the long term, sounds vague I know.
Look at the top PUAs, how many of them have a 10? I can't think of any
I don't exactly know what a 10 is. I don't believe in the phrase "Out of my league", and the phrase '10' is subjective. What can be one man's perfect '10' may not be what someone else wants. This even applies to a model who you'd have thought every man would like. Not true. There are men out there who don't like women who look like models (no, really there are!), there are some who like them to be a little plump and have an hourglass figure, some who like them to be small in stature and have small breasts (I myself tend to favour these), while some like them to be very voluptuous.
Most men can probably agree that some women are beautiful and some are not and be in agreement about which ones are/aren't. However one man's perfect 10 might not be another's, even if the woman in question has other options.
And here's a bit of a philosophical question. What is a '10'? Does it just mean looks, or is there something more? I mean you could have Miss Universe, but if there's no more to her than looks, then how can she be a '10'? Looks fade, but someone's heart does not. And you can have both, but when one goes, what is left? We all want different things, and the way I see it is IF I ever do find my '10', then it is probably going to be the woman I end up marrying. Like all people, she would not be perfect, but who wants perfection?