Well, what I meant (within the response length constraints of Twitter) was that we're not saying you can only buy it from us. Just that the customer needs to buy it someplace. That way a customer can do as the wish - ranging from buying a used one on eBay to getting a new one from Amazon or Best Buy.
Ultimately the objective is to ensure a customer is on a device that can (1) deliver the performance for which they pay and (2) is up to date technically (i.e. supports IPv6 and channel bonding) and is supported by the vendor (i.e. software updates & bug fixes).
One of the big risks we have to help mitigate is when a device goes EOL, which means no more software updates, and a security or significant performance issue arises in the future. By proactively beginning the replacement process this helps minimize any future impact when it is a major issue like that. So taking action gradually on a proactive basis prevents a more severe impact later on. In many cases, these are DOCSIS 2.0 devices and that technology and often the software is from 2001, the same year as the 1st gen iPod and when Windows XP was released.
Eventually a modem will go into End-of-Service (EOS) status. At that point there is a definite date/time limit for the device, after which it is de-provisioned from the network and the customer must replace it to continue service. This has been the case in the past with DOCSIS 1.0 and 1.1 devices for example, after years of work to encourage customers to replace them.
See also https://www.xfinity.com/support/articles/end-of-life-devices and the start of the EOL/EOS process for DOCSIS 1.1 devices https://www.dslreports.com/forum/r27473499-Speed-Heads-Up-Ti... and https://www.dslreports.com/forum/r28497383-Speed-Upgrade-You... and https://www.dslreports.com/forum/r30524429-Equip-Reminder-Pl... and https://www.dslreports.com/forum/r30450278-Speed-Heads-Up-Ti...