I don't think you have to show that the talent couldn't have been trained in the USA (or rather, it couldn't have been trained into USA workers), but that the talent wasn't trained in the USA so bringing in an outside worker is the only way to hire for the position.
You can't really expect a company hiring PhD's in a niche field to show that they couldn't have spent 7 years training an American for the work.
My former employer exclusively brought in that kind of talent.
But it was so hard to get the visas (and so much uncertainty in whether or not they'd be able to secure a visa for any particular worker) that they opened up a European and Canadian offices.
That's so funny. You realize there is already an O-1 visa, right? I hate to be a bearer of bad news but the vast, vast majority of H-1Bs are not PhD holders for which no suitable American PhD exists. If you go out into to the working world for awhile, you'll see that.
You can't really expect a company hiring PhD's in a niche field to show that they couldn't have spent 7 years training an American for the work.