It's a fact of life that most proprietary vendors only support their hardware for a few years with updates. Another practice is selling their cheaper modems with a lot of functionality missing, to encourage buyers to pay up for a more expensive model to for example get mesh networking, load balancing, real-time network monitoring stats, etc.
So it's worth always checking, when buying a new modem, if it is OpenWrt compatible, as this will allow you to reflash the modem with OpenWrt. It could also mean buying a cheaper modem now, and gaining some extra functionality at no extra cost. But most important is that you can continue to get software updates and security patches.
See OpenWrt - Wikipedia
OpenWrt (from open wireless router) is an open-source project for embedded operating systems based on Linux, primarily used on embedded devices to route network traffic. The main components are Linux, util-linux, musl, and BusyBox. All components have been optimized to be small enough to fit into the limited storage and memory available in home...