Consider this, the router is acting as a conduit, with no software required on the computer side. I just can't grasp how a network device, that only transmits network protocols, is incompatible with an OS.
I work in PC support, I have A+ and Net+ certifications (fine, they're just pieces of paper these days), I understand how computers and networks operate and the ONLY reason I can see an older router not "supporting" Win 7, is if you're talking IP v6, which is not in use to the point that it makes a difference in the home. If you're talking about wireless security protocols, Win 7 supports anything a home network would use so that shouldn't be an issue.
A router is a Layer 3 device, it operates independent of the OS on the PC. We're talking TCP/IP, TCP, UDP, all of which are supported by 7. If the PC can talk to the internet WITHOUT the router it should be able to talk to the internet THROUGH the router. IMO, and looking at the OSI model, you could take a 10 year-old switch, hub or router, plug in a PC with any OS and get online.
Show me something that PROVES something in Win. 7 will prevent it from working with an older Layer 3 device. Saying "it's not on the compatibility list" isn't enough, once a device is discontinued they will often neglect to update the compatibility list to show newer OS's.