The goal of pasteurization is to kill of harmful pathogens. If you do this early and package and store the milk in the right conditions it can be stable and safe for a long time.

If you don’t pasteurize the milk and leave it for a long time, pathogens in the milk, such as bacteria, can potentially produce toxins. Boiling it at the point might not help, no, as it kills the bacteria, but can leave behind the toxins.

So pasteurization is very effective if done early, but you can’t do whatever you want to the milk and then pasteurize it right before using it and everything is good.