I think a national security argument is much more sound than an economic one, although costs are externalized in a way that isn't obvious, i.e. ecological disaster that shipping everything around the world and back (components, assemblies) is, and hollowing out a local supply chain takes virtually no time while the impact or limits of it are hidden until abrupt breakage (i.e. covid-era shortages on basic supplies, wars, or heavy handed statesmen dictating preferential access to silicon or whatever today). That is, every nation has to maintain some stake in not hollowing out completely while still participating in global commerce.
> Economic efficiency would mean only a few vendors.
This is a massive lie monopolists tell you to justify anti-competitive mergers.
Increasing entity size has economies of scale (amortizing fixed costs over more units) and diseconomies of scale (bureaucratic overhead, long-distance transportation costs, etc.)
Economies of scale peter out with increasing entity size. Amortizing a $1000 fixed cost over 100 units instead of 10 saves $90/unit. Amortizing the same $1000 fixed cost over a trillion units instead of a million saves less than one tenth of one cent per unit.
Diseconomies of scale metastasize with increasing entity size. Entity size exceeds Dunbar number, language barriers and timezone asynchrony, corporate politics, independent jurisdictions imposing mutually-incompatible legal requirements, insufficient competition compromises incentives for efficiency as long-term incumbents succumb to Iron Law of Bureaucracy, etc.
By the time you're operating something at the scale of the entire planet, having the benefits of scale still exceed the costs of scale will happen approximately never.
Maybe.. and "a few" was pulled out of thin air, but these machines were national treasures with immense R&D expense. Think rockets, heavy aircraft, and lithography.. not commodities or software. Having stuff in these categories is kind of "you have to be this tall to ride the ride"
The history is complicated and a side quest to the conversation here, but a free market did play out in the US and became fever pitched after AT&T was hand tied in the 1980s.
A free market means if you can do something better/cheaper/faster, or at least convince other people of the promise, you have a shot. The more that come (think of a gold rush), the potential returns diminish so it will eventually be hard to either acquire revenue/customers or funding for speculative approaches if they are capital intense.
Once upon a time nations understood the issues better: https://en.wikipedia.org/wiki/List_of_telephone_switches