A 200,000 mile Crown Victoria is like a 800 year old house, or a 1,000,000 year old forest. It may look ancient, but everything inside has been replaced at least once.
The Crown Victoria / Town Car is not reliable, but its frame is robust, and its spare parts are cheap. That's how it can appear to last forever.
If you own a fleet (i.e. can plan for regular downtime) and have a dedicated maintenance service, it's a viable option. As a personal vehicle however...