"But I doubt you have ever been in a meeting where military folks were hashing out what they wanted."
Think again.
"Remember also, that they wanted a short, light weapon to replace the pistol for people who had other things to do than shoot the enemy."
Yes, I alluded to that. Had the .351, or a souped up version of the .351 been adopted, the resultant carbine wouldn't have been much, if any, larger, heavier, or bulkier than what became the M1 carbine. It would only have been a bit more capable.
"which would have made billions of rounds of ammunition worthless
Which is why MacArthur put the kaibosh on the proposed caliber change.
"and saddled the U.S. with an underpowered rifle in WWII."
Far less clear. As opposed to what? It was realized even back then that the old standard military rifle rounds, .30+ caliber and firing heavy bullets at 2,500 to 2,900 fps, were really overpowered for the kind of warfare that was developing.
Would the .276 have been underpowered? Compared to the extant military rifles cartridges of the day, yes, it was quite underpowered.
But, compared to actual battlefield requirements and experiences? I don't think so. Ballistically it would have been pretty close to the 7.35 Carcano round, and from all reports the 7.35 Carcano was a quite capable round in the limited fighting that it saw before the Italians had to pull the rifles back and convert them to 6.5 due to logistics issues.
In fact, the .276 Pedersen round was on par with military loadings of the 7x57 Mauser cartridge, which had made quite an impression on US troops during the Spanish American War.
Remember that the 7.92x33 Kurz round was, compared to the rifle calibers of the day, tremendously underpowered, yet the German experience with it was so positive that it, and the Stg. 44, were eventually intended to replace the K98k in active service except for very specialized roles like long-range sniping.
Probably the proper way of looking at this is that every major combatant in WW II went into the war with a main battle rifle cartridge that was significantly more powerful than what was actually needed. In reality, a waste of critical raw materials (copper and lead) and propellant.