There's no part of the merger that's good for America because Space-X enjoys government protection, and nearly holds a monopoly in orbital transportation systems, currently--and Tesla nearly holds a monopoly in electric cars. Generally monopolies should be broken up--not allowed to coalesce.
In studying American manufacturing, when we had it, those who filled a need with government and military supply were give special breaks that their American competitors were not, and that advantage stifled competition and created laziness and complacency in American companies--which in turn, left us open for the onslaught of foreign manufacturing, first from Japan, and then from China. If it weren't for the tilted playing field, perhaps we would have more manufacturing here.