Other cable providers (e.g. Comcast) charge you a monthly fee (~$5-15) to rent their modem. Buying a modem gets cost effective pretty quickly.
I think this modem "uncapping" thing has mostly been an urban myth. Maybe there were some really early cable modem systems in the 90s where that would have been possible, but AFAIK with DOCSIS the speed is regulated by the CMTS [0], not the modems.
[0] https://en.wikipedia.org/wiki/Cable_modem_termination_system
It depends in precisely what CMTS is running, what firmware it has and the ISP's willingness to replace/upgrade old hardware at great expense.
At the last telco I worked for we had about 12 CMTS's, and 2 of them were holding up a bunch of upgrades because of their age. One of the things they specifically couldn't do was provision the speed of the individual circuits.
Which works out great for me. I can use OpenWRT with no hassle.
More to the point, I see the cable stuff as "ISP land" in that it's directly interfacing with their internal hardware, and so has to dance to their tune very directly, whereas Ethernet and TCP/IP are common, and so will obey my rules in my home. I don't expect my modem to perform adblocking, which is why my router does it, and I'm not going to be stupid and try to "uncap" my modem to get more speed, so I don't see a point to being able to provide my own cable modem. As long as I can own the router which provides the only path in and out of my LAN, I can do everything I'm capable of doing anyway, as far as I can see.