It is not the buying power of large companies that enable good coverage at reasonable rates.  It is the spreading of risk across a large pool of people.  There is 100% participation in large employer health plans so the few high cost employees costs are spread over the entire organization.

Trump's plan is not finalized.  The fear is that young and healthy will leave Obamacare plans with the remaining covered people high cost claimants making those plans unprofitable and the premiums will soar even more than they are currently.  So the poor unhealthy old people currently in Obamacare plans will effectively lose their coverage.  Nice.

Paul Rand endorsed this initiative from Trump and he is a fan of the private market in an unregulated environment serving the needs of the American public.  It will not work as a large percentage of the population is baby boomer age that have high health care costs.  No private mechanism can cover that portion of the population profitably.