The rule, which has been well published and established, is that selection is based not on how you do at just one competition, but on average at multiple events weighted by importance. In my opinion, putting it all one one competition would be foolish, considering how results can vary so wildly from one to another based on so many factors. I'm happy the decision isn't made solely on nationals.
Gymnastics has been doing it this way for quite some time, and has gotten great results. A panel decides based on team strength. If someone has a freak fall at nationals but was otherwise strong all year, that one moment is not the be all and end all.