Comcast (and indeed other ISPs) doesn't guarantee speed. They are very clear to point that out in the teeny tiny fine print. They only real guarantee you get is a bill. Since there are no SLAs on home service, just be glad you get a connection at all. The "free" market says they have to make a reasonable effort to keep connections up and running, else they would lose customers. With Comcast 'growing' like they are, they have less incentive to keep the systems running.
keep in mind that the core infrastructure used by the power grid makes up a sizable chunk of the internet. not only is it used for commercial and residenrial Internet access but it is used for things like traffic light timing systems. with that in mind it can't just be unplugged. it has to br properly firewalled and segregated. hopefully that is being done and it has to be constantly monitored.