…followed by leaking capacitors on the board.
But, the oldest of these routers are approaching 9 years of age, ran continuously and considering the relatively high operating temperatures (>60°C), it is only to be expected to see the first hardware failures by now (also on the analogue side of the wifi cards). While I wouldn't consider it to be a general issue yet, chances to get hit by hardware failure (beyond the easy PSU replacement) are likely to increase.
Depending on your environment (climate/ air humidity), you might also observe corrosion on the PCB, as we've recently seen in a post about a Linksys EA8500; tropical climates or salty environments near the sea are a killer for any kind of gear.