Brewers sanitize, not sterilize. Sterilization is 1000x fewer bacteria than sanitizing.
Raw wort is a great growth medium. We just want out yeast to dominate. And it does so, very well. Like all microflora, it wants to create an environment that is hostile to anything that isn't itself. It does this by scavenging all the air, creating CO2 and not least, producing enzymes and ethanol.
A well pitched wort, that kicks off in a timely manner (12 hrs or less) has such an overwhelming yeast population that the relatively small population of other microbes largely get beaten to death by fermentation. What's left, usually doesn't have sufficient strength to do any damage to the finished beer under normal circumstances.
In a non competitive environment, bacterial will consume nutrients, protiens, starches, and sugars. Even at cold temperatures, they're inhibited, but not stopped. In such close proximity to nourishment, they will have plenty of opportunity to prosper.
In Lambic brewing, the time between entering the coolship and when fermentation begins is prime time for a lot of microflora that won't survive fermentation, but do leave a mark on the wort.
Acetobacter prefers ethanol, but will consume other wort elements, like dextrin. This critter is able to double its population every 20 minutes under the right circumstances. It will wait out fermentation before beginning its tour through your beer, if the population is sufficient. Long timeframes between chilling and pitching are good for acetobacter.
Even if you're going to blend the worts together, it's best to get SOME yeast into the wort. Even if it's just a pack of dry yeast.