You are confused. Friendship isn't an infection that destroys attraction. There isn't a magical window of time that once missed, friendship is the only option. Friendship isn't an investment that entitles you to more. Friendship isn't a currency that takes away someone's choice just because you are nice to them.
Don't undercut the value of your friendship by saying it only exists because you couldn't choose otherwise.
Rejection sucks, and it's difficult when someone doesn't return the same feelings. Grow through it, grow past it, but no one is obligated to be interested in return. Sitting in pain and refusing to grow is masochistic.
The philosophy that sexual attraction disappears the more someone gets to know you is a farce, propagated by those without the confidence to ever deal with someone not liking them.