All the conjecture I can already find seems to be based, mostly, off of the old habit of overshooting viscosity so it doesn't get too thin when it "wears down" - aka, shears. Modern oils don't shear like older formulations did so the idea of starting with a 40 wt. so it doesn't get "too thin" isn't a thing anymore. If in decades past you were using a 40 wt. so it only thinned to a 20 or 30 wt. by the end of its usage, if a modern 30 wt. stays a 30 wt... why use a 40 wt.? Add to that better neutralization of contaminates, better thermal stability, etc...

If these engines lasted so unusually long when they were new, on the crappy, ancient oils of the 1980's, why is it so hard to believe a current oil in the upper end of the quality spectrum (name brand synthetics, for example) isn't total overkill and one step thinner oil is actually protecting way better than say, Castrol 20/50 did twenty years ago?

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".