Cable Modem Upgrade

Recently, when I logged onto Amazon.com, a pop-up message told me that my device (a DOCSIS 3.0 Cable Modem) is no longer supported. I MUST get a device that is compatible. (I presume that this would be a DOCSIS 3.1 Cable Modem.) The next day, XFinity sent me an E-mail message, informing me that I must upgrade my cable modem for performance, reliability, and security improvements. It directed me to a product page offering new modems for over $400.

I don't have $400 to throw around. So, I went back to Amazon.com and searched for more affordable cable modems. I found loads of the DOCSIS 3.0 products still being sold. Some of them were on sale.

Question:  If XFinity is telling me that it's important to upgrade, why are stores still selling the older generation of products?