I'm talking about EROEI - the viability of doing it at all.
Private enterprise can play the capitalism game and make it cheaper. But it won't invent breakeven fusion to begin with. The risk appetite just isn't there at that level of investment.
It is possible we will get stuck and need another system to advance further, by going through regressions first.
The risk of going for a clean energy project is massive. It can fail completely, it could work but not return much profit, there are lots of ways it can go wrong.
However - the problem here is that the 'do nothing' state is not static utility. Do nothing results in everything going to shit, with very little variance.
Our capitalist systems aren't really set up to deal with that, it's a tragedy of the commons. That's why you need things like taxes on oil - to force the utility function to accommodate bad things.