I completely agree that the primary weapons of warfare are the spear and the arrow
and consequently the most efficient armours would deal most effectively with
these threats. But efficiency often took a back seat to fashion in my opinion ...

On another slant ;
When looking at helmets especially we can see that archaeological finds
from the sixth century to the tenth mostly comprise grave-robbings of aristocrats
(Vendel, Valsgarde, Tune, Sutton-Hoo, Broa, Gjermunbu, StSeverin, Frankish graves)
or at the very least high-ranking professional warriors ...
Late-Roman-period helmet finds (let's say fifth century and back)are often also
high-class but do include poorer evidence from excavations of forts and camps.
In this regard the trend in actual FINDS has been from poorer quality upwards ...
rather contrary to the outdated assumption of degeneration from Roman eras.
(Also, segmented Roman armour can be a real b*tch doing anything very athletic
like fighting or ...riding a horse. Forget that for me! Ringmail is so much nicer.)

So I'm saying that quality may have improved, it certainly did not decline from
late-Roman to early Viking-period.

I'm also NOT saying quantity declined, I am afraid I haven't seen much evidence for
this. What I have seen is belief that because all the Roman soldiers are supposed
to have been fully armoured, and because later armies are supposed to have been
only partially equipped with armour then armour quantity must have declined.
I'd say maybe yes maybe not, ...

On the other hand - how much roman armour has been found? AFAIK, comparativly small amounts of roman armour has been excavated, but it doesn't prove that only a small fraction of roman soldiers would have had armour... Then one has to remember that it's easier to find roman armour if excavations are made at the sites of old roman forts, were thousands of legionaries were stationed for a long time. It's much more difficult however to locate such concentrations of fighting men dating from the early Middle ages.

I wonder if it is possible to devise some tests to determine what
may be more effective at resisting side-of-face blows ;
Hanging mail curtain/coif with and without padding
or Cheekplates of various designs with and without padding.
Are there any remains of padding on surviving cheekplates?

Mikael you are right, it must be very difficult to find any concentration
of rank-and-file armour from migration/dark-age/pre-viking context.