I also think that the supernova crews were on a similar level. By going to the new world they were forced to improve rapidly or perish, it's a highly risky venture. If they run into a major powerhouse too soon they have no chance. The strawhats could have gone the same route and survived depending on their luck, but their luck hasn't been all that great in the time period before that so staying out of the picture for two years is a lot safer. In some respects I think this way allowed them to improve faster by focussing on certain aspects of their abilities. For example, I doubt Luffy could have mastered haki as quick, without the expert guidance he got now. Any surviving supernova will be a force to be reckoned with though, can't wait to see a reunion.
