Heavy users are not the cause of network congestion

In peak usage periods the congestion caused by a heavy downloader is the same as congestion produced by every other user who is downloading at that time. No more, no less.

Outside of peak usage hours, the heavy downloader is simply creating value from a resource that has already been paid for and would accrue no value to anyone if they stopped.

Just as each and every car in rush hour is contributing the same amount of congestion, thus in peak usage periods everyone using their connection is contributing to the same amount of congestion. Just because one car (or one user) may have travelled 100's of miles not in rush hour (or downloaded 100MB's in periods of low network usage) this does not mean they create more congestion in rush hour than the car that only does a few miles per day but always in rush hour.

If you use the network during peak times you are creating just as much congestion as the heaviest downloader. If you use the network in off peak times then the issue of congestion does not affect you.

A generalised download cap does nothing to address the issues of peak use congestion, as those "heavy users" are still free to use their connections at such peak times.