I've had my 4.4 for 10 years now. It's had timing chain guides done, transmission rebuild and all the suspension components refreshed in my time.
Just had a new ac compressor done too.
Yes it's 21 years old but it's used every day as a general work horse but I keep it in good condition.
I keep looking at Cayennes (had on) and Ml63's to replace it but just can't seem to part with it.
I also rarely see another on the road and if I do they stand out from the crowd.
So the answer to your question from me would be to sell only if something becomes available that is better than what I have already which I have not found yet.