So. How much of a rebuild can you do on a car and still call it the same car? Some places I've heard of having to remodel 1/6 of a home at a time to avoid zoning issues - because anything more and it has to pass current zone rules.
So. New block? New transmission case? New cylinder head? Body panels? You could just about keep anything going for a long long time. OK. You need something that was popular so you can find parts.
Hmmm. W123? Well, most people get tired of the same car long before they really get all the use out of it. Then some kid gets it who revs it in neutral and drops it into drive (oh, that's another thread).