"A decently trained Infantry Battalion can do anything a Ranger battalion can do -- and at far less cost; give any Battalion the training time, gear and money a Ranger Battalion has and he'll be close enough in capability for government work." (emphasis added / kw)
Your cite of Slim is apropos -- and that comment by him is one of the reasons I named him in my list on the Great Generals thread. Slim 'got it' -- pity about some of the others...

Only thing I'd add to your excellent post is that while some airborne capability is needed, most missions do not require that capability; what they do require are just well trained and competent units that are trusted to do missions without over supervision. The MEU(SOC) was a good start...