Really?
Some of you have never had routers wear out?
You are indeed lucky people.

I would agree that 3-4 years seems to generally be around the maximum life in home use.
I would agree that they should last forever - no moving parts etc - but they all seem to run quite warm and start going flaky after a while. As Dairyxox says they often have poor ventilation and end up being placed in quite warm spots - certainly in domestic use.
Have had several just lock up and die over the years - worse still when they just start behaving erratically. I dead router is better than a wobbly one.

I had a TP-link router that appeared to have plenty of ventilation slots on it - it was always very hot on top. One day I noticed that the ventilation slots were just plastic ridges and realised there was hardly any ventilation on the box. Internet traffic seemed to start going quite slow/hanging when it got very hot. I (carefully)used Mr Black and Decker to add a few extra 'heat elimination portals' to the box and it made a huge difference.