Many good points here. I am in the minority apparently, but I would keep it running for two main reasons: 1) the price of replacing it with late-model used is high (often > $25k), and with new is outrageous (often >$50k). Used may or may not have been maintained well, you probably won't know. So you could end up spending even more. 2) I do all the upkeep myself, so the $ pencils out and I know that it has been well maintained throughout its lifetime (single owner). However, if you have to have work done by the pros, it is a lot more expensive and might not be worth it. 3) The rest of my truck is good - no rust, always garaged, no towing or abuse, trans is good, interior good, etc.
Just a few points for you to consider. I have thought a lot about this too as my Exp gets old. Again, the outrageous pricing of new or late-model used Expeditions is a major factor with me. Good luck.