The 6509 and 2811 and other older models are amazing. I use catalyst switches and routers I bought from ebay and craigslist almost 10 years agi that were probably resold several times after production use. I am no longer in networking but I was always impressed when I ssh into some arbitrary switch or router to troubleshoot and see 10+ year uptime. I bet 20+ is common these days.

I am sure others have stories in this regard. Someone told me their remote router died after 12 years uptime but only because of a lightning strike.

From a software perspective, the Cisco IOS has been doing "unikernel" before it was hyped. Can't say that's a factor but having a standalone binary as the entire OS might perhaps make testing and shipping a sturdy stack easier?