I contracted for Xfinity 1 Gig service with the Docsis 3.1 availability now. I expected that they would replace 'all' the old coax that had been installed when we were Comcast customers over 25 years ago. I live in Utah and I am sure that the cable has weathered during all that time. With the Docsis 3.1 capability dependent upon having a healthy coax. When the tech came to do the install, he looked at the existing 'street feed coax' and said that "it will do" and then ran a new coax from the service box to the location of the cable modem. Why not the whole length? From what I have read the Docsis 3.1 speed can only be achieved by using the newer, larger Cat 6/RG6 size wire. The coax that was installed in 1995 may have the correct designation; but, I would bet that if you did a 'signal carrying capacity' between the 25+ year old cable and a new one, you would find a difference. The Comcast 'speed tests' that I done show between 650-700 Mbps. Not good enough for a 1 Gig. service.
SO obviously my first question is if you plug a gig ethernet into the cable model and go to dslreports what are you testing at?
If they replaced everything in your house as part of the install then I'd be happy. They are responsible to fix everything outside your house if it fails or doesn't meet specs. Complaining about that is like complaining they didn't also upgrade the hardware at the switching station.
The Coax in my house is 20 years old and I still get solid 900+ on wired connection.
The line from the pole to my house was installed in 1995. It took three service techs to come out before someone decided to take a good look at the cable. It was worn, had cracks in it and the 'strain line' was separted from the data line at points. After the cable was replaced, my speed jump 400 Mbps. Don't assume that a cable will last a lifetime.
If it didn't work then of course they need to replace it but your original statement was that you'd assume they'd just replace it. Either way sounds like you got it squared.