The later specs allowed for floating channels based on channel maps, which allowed Comcast to bypass those degraded channels.
Note: I'm not an apologist, but I worked for Comcast and for a subcontractor. Comcast treated (at least in my opinion) their customers like wallets that called and complained, but under the subcontractor I saw that since they didn't rewire 100% of all networks purchased, it was common that the older lines were causing the degradation and also reflection on other RF channels sometimes on the other side of an area even. Now if Comcast invested in their network as opposed to buying other companies and calling it investment, this might have been fixed, but that would be decades vs. having every modem that wasn't compliant to the new spec swapped.
The newer modems support more channels and newer modulation/technology. This isn't just about supporting newer speeds. In order for them to support those newer speeds for other customers they have to upgrade their equipment to support more channels and newer modulation/technologies.
At some point these older technologies are not just wasting resources by being less efficient, but are preventing the company from upgrading their equipment.
The reason I don't understand, is because it's common to see people complaining about the state of broadband in America compared to other countries. Yet Comcast is probably the most progressive as far as pushing the technology goes. Don't misunderstand me, I believe Comcast holds a near/total monopoly in many locations around America but at least they're progressive with their network and technology despite the lack of meaningful competition.
it's as easy as walking into a Comcast store and swapping it
I live about 6 minutes from one, and it can still be a multi-hour adventure.
I guess this approach does not scale as well as the 400 lines of Javascript!