If the pipes you have are made to serve 1000 people and you have 100000 customers, there's something wrong.
As you said peak hour is peak hour nothing ca be done about ti the usage is not really elastic, so if congestion is a possibility then congestion there will be.
off peak the guy watering his rice fields is not bothering anyone since the pipes are there and filled with water no matter the usage. Besides that guy has an upper limit on how much water he can draw per second, a limit he cannot go over no matter what (removing caps on modems is ground for service termination at the minimum).
caps is having the cake and eating it too. plain and simple.
Some ISP do impose caps (account for usage) during peak hours and have reasonable limits (around 300GB) off peak hours (during night usually) download/upload to your heart's content. Still asinine but reasonable to some extent !