Yet, it is reasonable that people expect x % of their traffic to use IX's. If those IX"s are gone then they will need to find another path, and may need to upgrade alternate paths.
I guess the question is.
At what point does one build redundancy into the network.
No, it doesnt necessarily use IX's, in the event of there being no peered path across an IX traffic will flow from the originator to their upstream "tier1" over a private transit link, then that "tier1" will peer with the destination's upstream "tier1" over a private fat pipe then that will go to the destination via their transit private link.
I'm only aware of a few providers who transit across IX's and I think the consensus is that its a bad thing so it tends to be just small people for whom the cost of the private link is relatively high.
I think you are missing a one critical point - IX in this case is not an exchange. It is a point where lots of providers have lots of gear in a highly congested area. However they connect to each other in that area does not matter. Now presume those areas are gone (as in compeletely gone). What is the possible impact? Alex