Then along comes the FCC, and says they don't get to own and control the network end-points anymore, thereby throwing a massive wrench into the works, and messing up the assumptions upon which their network is architected. Large complex systems don't just magically change overnight because of a governmental decree, despite how much legislators and administrators might seem to wish so. It would take a great many man-hours to rebuild their current system to comply with the update.
How many? Tens of thousands? Hundreds of thousands? More? I have no idea, but I do know it ain't gonna be a small number, never mind the monetary costs. And that's just the capital investment to get compliant. Ongoing support is also going to costly. And you can be confident that Comcast would then pass those costs on to their customers in the form of higher subscription fees, and high bring-your-own-device fees (or a corresponding use-our-device discount if the FCC prohibits the former).