That's true, I did upgrade from a 3-4 year old DOCSIS 3.0 Surfboard to a 3.1 Surfboard at the same time. I didn't run any metrics to see what actual bandwidth usage was, I was just waiting for symmetric gig fiber at the time.
Outside the US coax based internet is rarer (its fibre or copper) so there isn’t a lot of good modem/routers.
So if you can’t get fibre, get an Eero or similar and at least replace your router.
When people replace modems they often get an upgraded one that can support more/better channels or DOCSIS versions. These new channels usually are much less contended (as only a tiny fraction of people have them at first), so it seems like a huge improvement, then as bandwidth demand grows and more people get new modems which can support the additional channel it begins to degrade.
However, I would say that with DOCSIS3.1 and DOCSIS4.0 you are starting to get to a point where there is so much more capacity available and bandwidth growth has slowed down so much you are starting to see a lot less capacity problems IMO.