We might be mixing who "MB" is. I haven't seen anything in the service or maintenance manuals that says to change the chain at 200K miles. I haven't seen anything that says to align cam marks and read off the crank pulley as a means to determine chain stretch. Lots of sage advise from experienced techs and DIYers have been taken to be gospel... and with good reason.

The proper way to measure chain stretch is in the manual. It calls for instruments most people don't have. Eyeballing by an experienced eye is probably a reasonable shortcut. Taking 5 readings is an excellent way to eliminate variables. How about getting another pair of eyes to agree that the timing marks are aligned? Just to eliminate an area of subjectivity.

To your point, it's exactly the case that proper maintenance will keep these engines in top shape for a good long time.

Sixto
95 S420
87 300SDL