1) You pay $40/month for an unlimited 10Mbps connection, but can only get 10Mbps at 2-4am in the morning. Other times, because of high network usage, you get an unstable connection that goes 3-5Mbps, or even slower during peak times.
2) You pay $40/month for a 10Mbps connection with a 100GB limit. Most of the time, your connection speed is around 10Mbps, but you just need to watch how much you download. There is a tool provided for you by the ISP to check your usage, updated daily.
You forgot an option:
3) You pay $40/month for a 10Mbps connection with a 40GB limit., but can only get 10Mbps at 2-4am in the morning. Other times, because of high network usage, you get an unstable connection that goes 3-5Mbps, or even slower during peak times.
The problem is you are starting with a questionable initial premise. You are equating total monthly usage with maximum use at any given moment. The two are not necessarily directly related.
There are bound to be certain times which are most heavily used, simply because those are times when people can be online using it. Sure a cap might make someone decide to forgo a download of a large game from Steam or their consoles online store when they could just pick up a copy instead. But when they are using the internet will still tend to be during those peak hours.
The degree to which it restricts use (a bad thing) will be greater than the amount it reduces congestion (a good thing). A large bad for small gain isn't a good trade.
To make the obligatory car analogy, if your town taxed its citizens based on monthly car mileage, would that be effective at cutting down on rush hour traffic? Or would people still drive to and from work, but just cut down on driving for non necessities? How would making the roads a ghostland at midnight help you get to work faster on monday morning?
The fact is most ISPs in Canada have put in caps in the last few years. The holdouts were the independents who piggyback on Bell and this decision as you seem to understand was really about trying to limit them by taking away the only thing they really had left to differentiate themselves with. So when you talk about the magic which will happen, sorry, if it was going to happen it would have already. It didn't.