A car that reaches 1M miles with 100k a year is very different than a car that reaches 1M miles in 80 years. Also a special purpose vehicle that cost a lot to replace/design/build is definitely worth saving rather than replaced. Miles is not the only thing to look at.
I'm already at 220k but I think the amount of HWY miles I put on in the last several years justify keeping this car for 300k. Now my in law's 93 Escort is beaten to death and has only 80k, but I don't think I want to keep it for even 150k.