When Kenny Roberts went to Europe and kicked everybody's ass in 500 GP, he was a product of the AMA Grand National motorcycle racing program. To be number one you had to race flat track, mile ,half mile, short track, TT and road course, this made for very skilled riders, the group that followed him over came from the same series. When AMA split the road racing and made it a seperate deal America lost it's edge. Couple that with Kenny opening up his racing school in Spain, teaching riders American flat track racing, then putting them on mini road race bikes to get them comfortable sliding the front getting in, and sliding the rear getting off, it changed GP racing. By the way, Spain has kept that program going, to great effect.
I agree with what you're saying but IMHO the American "Golden Age" of 500cc GP was after Roberts with Lawson, Rainey and Schwantz. Those guys weren't products of dirt track but rather went mostly through road racing ranks. Doug Chandler was a supremely capable dirt tracker but when he got to the Big Show he wasn't able to make a dent.
In modern times, Nicky Hayden is certainly in the mold of KR Sr. - dirt-track upbringing followed by domestic road-racing domination. However, he was only able to compete at the highest level back in the 990cc days before TC. He hasn't been able to be competitive since the 800cc/TC days until the present.
MotoGP and road racing now has become so specialized that it has evolved past those "analog" roots. Young riders now, aside from the raw talent, need the new skill set of working with a team of engineers and those that are able to translate their needs of the bike and chassis to the team are the ones that will be at the top. Not sure what type of farm team that needs but certainly coming from the most bike-racing-centric countries helps.