There's at least one way to make IXPs to handle nearly arbitrary amount
of traffic; so this is not a fundamental problem. It's only a matter of
time and money. http://www.pluris.com explains how it can be done.
In other words, traffic is not the problem. It is more economical to
have fewer high-capacity facilities instead of a zillion of lower
capacity ones. BTW, the number of switch sites LDCs have is a lot less
than the number of their POPs, for exactly that reason.
--vadim