No, that isn't a realistic way to look at things. When you order a specific amount of bandwidth it is to support a need. What percentage is to cover peaks vs average loads should be disclosed, but it isn't really material. What is material is that an ISP has adequate bandwidth to support its users needs collectively.
So, how does the ISP deal with customers that have a disproportionate usage profile? Options I see are: pass costs on to other customers, drop subscriber, charge subscriber more, or throttle subscriber to be closer to normative usage. Make less profit is not one of the options.
What regulators (or someone) need to ensure is that changing average needs are not treated the same way-- that the ISP does not simply stagnate because they do not wish to invest. That is the obligation of a monopoly/duopoly.
Now, what percentile constitutes abusive customer is a valid question. I would imagine it is something on the order of the top 0.1-0.03%, but I don't have data to justify that.