>SpaceX _received_ a lot of tax money
Yes, but most of that money was either for doing resupply missions to the ISS for developing the capability of tranporting humans to the ISS. The resupply missions to the ISS are cheaper than what NASA can do themselves, and SpaceX transporting crew is likely to save money compared to Russia doing it (because Russia raised the prices as competition disappeared). So in a very real sense the government is gaining money here by spending less on something they need anyways.
>There's also a lot of industries in which the capitalist model produces something much less efficient than alternatives. Public transport
It's rare to have a public transport system set up to even allow for fair and open competition. Most started as state monopolies and transitioned to private monopolies (which are hardly capitalism). The areas of transportation where the capitalist model actually happens seem to do great (for example airlines, with the possible exception of the united states where mergers and consolidation seem to limit competition).
>This is, I think, a very dangerous argument. If the existence of megacorporations was taken to its logical conclusion, with a handful of monopolies, where workers rights were removed and all the power went into the hands of the companies, _this would still be true_.
Just to be clear, I am against monopolies and for government-ensured workers rights. But in a hypothetical scenario where workers rights vanish, workers still continue to be a limited resource. If I offer a job, but everybody willing and able to work already has a better one I won't find a candidate. I have to make the job desirable enough to outcompete everyone else looking for employees (for example by raising pay or improving working conditions). The more jobs offered on the market, the higher that competition.
With higher unemployment of course the bargaining power of the individual emplyee sinks, and with that the working conditions and pay. But high unemployment also means higher social unrest, which means the state is incentiviced to provide sufficient benefits/insurance/whatver to keep the unemployed happy, simultanously giving the employees another way out of entirly undesirable jobs.
The true danger is if companies try to find creative ways to make their job "attractive" without actually benefitting the person employed, or if they manage to prevent employees from switch to a better job. That's where exploitation begins.