I don't think 20 years will happen but I've seen modem/router combos that lasted for ten years. Customers bought them way back when, and never considered buying replacements because they just worked. If all you do is read news and do taxes, you can still get away with 802.11g easily. You can pull about 10 to 20mbps down that, and for many people that's more bandwidth than their ISP provides then with.
Why buy an 802.11ax capable router when the best connection you can get is an 8mbps DSL connection that drops out when it rains?
Even at that price it was cheaper to rent a modem for a buck a month, but some people like to own things, I guess.