True, there would probably be someone with a dial-up modem or satellite dish internet. Rarer would be someone who has both that and the wired internet that has the outage, but if it's a big enough segment to matter, out of a million people there's bound to be a multi-home geek.
ISP network cuts are just your local area. If you still have communication with the rest of your area, it would probably be something like 1/1000 of the world or less. Block generation in the segment would take several hours per block.
I favour the plan to monitor if the frequency of blocks received drops too slow. That covers a large range of possibilities.