I agree, start with the best 6.5mm and go from there....but that’ll never happen. Not with the way our screwed up system works. Just as the 1930’s supply of on-hand 30-06 caused the army to dump the best round for the Garand, today our “legacy” 5.56 and 308 causes the army to stick with those rounds no matter what.
And it will forever, I suspect.
Historical footnote to legacy calibers: Roman coins were copied in size and weight, empire after empire, down through the centuries to the Spanis “piece of eight” coins, to the original American silver dollars.
It was always just easier to say, “Let’s make our new coin the same as the most common old coin still in circulation.”
Thus it is with rifle calibers.
Up to a point. When we get our hat handed to us by adversaries who use a technological advancement, it brings change along in a hurry. Examples abound, from the reservist volunteers of 1898 going up against the Spanish in Cuba with their Civil War Springfield muskets reworked into single shot breechloaders, versus Spanish 5-shot Mauser bolt-action repeaters firing smokeless cartridges. The good news was it gave us the Krag and Lee repeaters, whose shortcomings were resolved with their replacement by the Springfield bolt action rifle of 1903.
Likewise, the M14 and early AR15 rifles were thought to be just dandy when fitted with a 20-round magazine, since that was what the WWII [and late WWI] BAR had used. And then US troops started going up against AK47s with 30-round magazines.
Our present enemies have mostly fielded that same weapon, so our shortcomings in the small arms department aren't as critical as those in personnel body and vehicular armor were. And it's always the squeaky wheel that gets the grease.
But either a breakthrough in propellant technology, probably caseless, or a situation in which American troops find their most basic tool to be horrible inadequate will get great bright spotlights of attention on the problem. I would be happier if it's the former.