The reason everyone is freaking out is because they feel pretty darn strongly that the ISP should not be injecting code into webpages delivered, especially not in an automated way without some oversight. If this is to be a service, the bar for what is necessary for such information must be far higher than "an automated system decides it's time." We get into really scary territory just by doing this in the first place, but to use it for advertisements or basic maintenance? That is a misuse of such technology.
And no, I don't think people would be as livid as you suggest if the modem just broke; ISP modems are fragile little things, and it's not uncommon to go through them. I don't think I've had a single ISP where I didn't have to eventually, and the natural progression for each one (Comcast included) was:
1. I called the ISP
2. We did some test with support
3. Once we did the Speedtest / reboot song and dance, a new modem was issued that day.
This is expected; if I had asked for such a service from Comcast, this would be a different discussion entirely (an Opt-In service), but as it is, it's a pretty lame reason to suggest that Comcast needs to be able to inject data into pages I load.
And I rather liked Comcast for the year I had it - I wasn't keen on being on them since I would rather have been with our Municipal, but the place I was at was not yet in a service area for the municipal. More or less, even with my support and canceling experience, I was fine with the service I received. This would have upset me considerably.