Well, as far as metric not taking off back in the day there were quite a lot of metrics around in the good old days in the USA. I forget if it was the Win 54 or 70 that was chambered in the 7.65 Mauser, and the 7x57 and 8x57 were both available in Remingtons or Winchesters, I forget which. There were also some "Americanized" metrics around although I forget just which ones they were. Of course this is back mostly pre WW2. After WW2 the 6 and 7 mms kinda took off. The 243 and 244 were 6mm's and wiped out the 25's and the 7mm in all it's varieties did in the 270 to a large degree, there were a couple 8mm too. And of course the 9 and 10mm handgun rounds took off. As far as price, yes, a new Rem or Win was a lot more than a surplus 03, but a surplus 03 was a good deal more than the surplus Mausers, Arisakas, Carcanos, etc that were offered through the mail. I've seen those ads for $1.50 Krags too, and 45 Colt New Service ex-NYSP revolvers with the holster for about $12,00 and that was in the 50's! But you are right, metrics never appealed to the US buyer compared to an "ought six".
Pretty much all my older loading manuals from the 30's on forward into the 80's anyway carry language about certain rounds having to be loaded to a lower pressure due to older surplus rifles or black powder actions, or you will find that the loads are just pretty tame across the board. I've even got one someplace that says the Arisaka is "unsafe" to reload for and should not be fired at all. Obviously pre-Ackley testing! Same with anything to do with the 250 Savage having to be loaded down for the 99 action, yet no one ever said anything about loading down the 99's in 243, 284, 300, 308, 358. Thoughts and times change.