If we're using a car analogy, it's actually more like knowing when to trade it in.
With LMA's FA clock ticking, we were like the guy whose truck has been running like a champ and is now at 98k miles on a 100k powertrain warranty. Sure, you've upgraded a few things here and there, had to replace a broken mirror and tune up the cruise control, but the engine's never had a problem. It's Labor Day weekend and dealers are desperate to throw deals your way. You could trade it in on a sweet 0% down deal on another new-model truck that may or may not end up not being as reliable, or may get t-boned the second you drive it off the lot, but if you don't you're running the risk that your truck is going to do what most trucks do right after their warranty period ends and have the entire engine block blow and leave you with no truck, and nothing to turn in except scrap metal and parts scavenge.
Historically, the vast majority of UFA all-stars leave (consider 9 years the warranty period for keeping your good players), and the majority leave for nothing but cap space/trade exceptions and some 2nds. Those who are traded generally bring back some good deals.