You ask a valid question. And it's a difficult question to answer. I think you could look all over the world and see various answers.
That you think Trump had any kind of insight on answering it, at all... sigh. Most states are already a large enough risk pool that going larger is not going to unlock any value. All that "eliminating the lines between the states" would do is cause all plans to re-organize into the states with the lowest levels of regulations.
If state level regulations were actually driving up costs, then each state's government should double-check their own. It's telling that the people selling that line never specified which states had excessive regulations or what those regulations were.

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