Infections take longer than 3 weeks when infectuous populations start out at such small levels. Keep one for a few months and then open it up and tell us.
Lactobacillus, Pediococous, Acetobacter etc are all very slow and those are just common ones that like to live in beer. There are a lot more different nasties that can get in that will take longer to ruin your beer and turn it into vinegar.
Brewing, bottling, and aging beer is something that has been done for a long long long time, if this were something that was a good idea everyone would know about it because someone probably tried it at a brewery in London or Germany 600-800 years ago and it ruined beer because its a massive vector for infection. Things that DID promote longer shelf life and stabilized flavors in beer are things that spread VERY rapidly in the industry. When hops were discovered to do good things to beer and especially longer shelf life nearly the entire brewing world adopted this very rapidly (relatively speaking).
Even modernly speaking, there are brewing scientists who do research all the time for different things both natural and more synthetic to help improve brewing and beer in different ways. Brewing beer isn't a mystery, its science and things like infections and sanitation are fundamental. Adding raw oats is an unsanitary practice that WILL ruin your beer over enough time.