I'm just about to retire my 1996 Trek 520. I'm not sure how many miles on it, but I'd guess close to 30,000. The bike is still going strong, but the type of touring I'm doing these days has changed. I'm doing lighter/shorter credit-card style touring and equally likely to be on what these days is called "gravel" vs. all on road. Another consideration is reducing my overall number of bikes (along with other stuff.) To be honest: the reduction in bikes is also an excuse to play with new technology, like a 1X drive train.
So, I've decided to do a "N+1-2" - I've added a Jamis Renegade to be my touring bike plus "gravel bike" for those non-paved rides, or rides with my wife. I donated my 1990 vintage Schwinn hybrid to Bikes for the World - that was my beater bike/gravel bike equivalent and what I used for touring on the C&O Canal Towpath, Great Allegheny Passage trail, etc.
I'm still not sure what to do the 520. Bikes for the World doesn't seem to be the right fit for a touring bike, ACA didn't have any good ideas for how to donate to someone wanting to get started, so still looking. Right now, it is my Zwift bike down in the basement, but that means I have only done a N+1-1!