Who’s going to buy the US-made components if the manufacturers that use those products have already moved to China because of the tariffs?

Probably those same manufacturers, depending on whether the added labor premium is less than the cost of shipping finished goods across the Pacific.

As a first-order approximation, transoceanic shipping is essentially free. A quick google gives me $5,000 to ship a container from China to a US port. Considering the volume that a 40' container can hold, yeah, that's pretty much free.

Compare that to even cheap US labor rates and it's a no-brainer.

The stated goal only matters insofar as it affects the policies that are created.

In 2018, it's a whole lot easier to avoid doing any manufacturing in the US than it is to avoid using any components made in China; and both approaches avoid the tariff as written.

If the goal is to bring back component manufacturing, they need to go back to the drawing board.