Well, at that point, if the current graph charts cost per gigabyte and approaches zero, then you would change the scale of the graph to show costs in terms of terabytes and eventually petabytes.
Ultimately, after traversing enough benchmarks, the scale would go logarithmic, and measurements would have to take orders of magnitude into account.
Last time I checked, ISP does not equal e-mail domain. If I recall correctly, ISP has a specific meaning: "Internet Service Provider."
For example, AOL and Comcast are ISPs, but gmail, hotmail and yahoo are not.
Maybe you receive an allocation of e-mail addresses when you sign up for internet service, but that doesn't mean that the domain of the e-mail address represents internet service.
I think what we're seeing here is that the higher age of comcast users is an expression of the fact that those are the people who own the homes (have their name on the mortgage and deed of the property) or sign the lease and pay the rent, and thus also are more likely to pay the broadband bills, and are more likely to use the ISP e-mail address, since they're the ones looking at the features their ISP provides as part of the internet package they sign up for.
Meanwhile, the kids don't pay the bills, and are probably suspicious about whether their parents can call the ISP and demand the password, to snoop on their mail, and thus tend to avoid the ISP domains.
Ultimately, after traversing enough benchmarks, the scale would go logarithmic, and measurements would have to take orders of magnitude into account.