The actual issue which is often blamed on routers/modems is that DOCSIS can be horrendously overcontended, especially on the upstream. It can really vary depending on the CMTS you are on, which means it can change street by street. When the upstream is contended your TCP ACKs get lost/delayed which absolutely hammers performance.
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.
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.