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.