I am surprised that ISPs haven't already built up terminal service farms and started renting out thin clients to grandmas. You get a thin client computer installed and setup by a tech, a fully managed desktop with most of the common software you need to get on the Internet plus they can sell you space to store your family photos. Grandma doesn't need anything more than a web browser, Office Online, and the Microsoft freebie sites.
An ISP could do the same with an LTSP solution and Google Crome and Google Docs but it is just way easier to find people who have set up Citrix/terminal server farms.
An even better solution would be thin provisioned virtual machines. If the ISP controlled so many of the computers I feel you would get that 100 Mbit link a whole lot faster.
Back to the mainframe days. And yes, I know there are lots of barriers to this type of solution and it severely restricts grandma from running all the stupid apps she thinks she needs, but that is not the point. The point is why aren't ISPs looking to tap this market? It is there.