At some point in all this is the idea that subsidies get around otherwise-insurmountable coordination costs, where the other actors lack the resources to take action that would be to all their benefit. Back in the day, this might have been the case for these giant projects, e.g., could private industry have managed the telegraph, or undersea, without subsidy?

At this point, with many companies that are effectively as rich and powerful as nation-states of old, perhaps what were sensible arguments in the past are no longer relevant. Presumably this has been explored in depth someplace, but I don't know where.

I think so, because financial markets realized the advantage was too great to ignore.