... so fighting a counterinsurgency... you could shoot all day with 5.56, 7.62, .338 and .50 at well dug in Taliban ...
My emphasis, obviously.
Ah, but now you are moving the goalposts. You prior statement was much more absolute.

And even in your example the small arms fire was still
important, wasn't it? To fix the enemy until you could drop something on him.
And for obvious reasons we're never going to have good data on casualty generation for insurgents. I'll look around, but I'm not optimistic.
All I'm saying is "nontrivial." If I may indulge in a reductio- would you propose arming our infantry with revolvers? Obviously not. The rifle has to be a credible threat for infantry to fulfill it's function. Otherwise, why the hell does everyone still field infantry?
If OTOH you're trying to say that "
absolute lethality is not as important as
casualty generation", and that thus
within certain minimal limits a truly powerful caliber is moot, well, yes there's an argument to be made, there. I can grant you that, if I had misunderstood your point until now.
Because that argument is almost metaphysical, if for no other reason.
EDIT-- Just FYI, percentages of small arms casualties from other wars- some non insurgency, since you asked- though relevancy is questionable:
51% Germans in WWI
30% Germans Russian Front WWII
33% US Army Bougainville WWII (25% rifles, 8% machineguns)
31% British Normandy campaign WWII
33% US Army European Theater WWII
20% US Army Mediterranean Theater WWII
52% US Army SW Pacific Theater WWII
44% US Army Pacific Ocean Areas WWII
27% US Army Korea (there's a
very large "unknown" fraction in that data for some reason)
42% US Army Vietnam on Search & Destroy missions
16% US Army Vietnam on Base Defense missions
24% British in Northern Ireland
23% Israelis in Lebanon 1982
55% US Army in Somalia
These are
all battle casualties- i.e. including wounded, not just fatalities. The small arms fraction tends to be
higher for killed than for wounded. (None of these data include noncombat casualties.)