I wasn't really that serious, but you can't say Bard won't do that until he doesn't. You can't say Joba will dominate for years to come until he does. You can't say that the Rays will win a World Series title until they do.
I'm not saying anyone has been as good as Rivera for so long. I was just using his name as an example of how I won't call Bard the Sox closer of the future until he's pitched more than 20 innings.