The grid upgrade can cost millions for a DC fast charging station.
The inverter to make the DC power can cost 100,000.
Why not skip all this and have a diesel engine coupled to a 400V DC generator, thereby skipping the inverter and grid upgrade?
I think this would be cheaper, and lower capex and operational costs. Not to mention, most streets have natural gas lines which could power the diesel engines with a cheap and clean burning fuel. Natural Gas is very cheap around 4c/kwh. A 20% efficient generator, would yield 16c/kwh.
Probably the same reasons the vast majority of electricity users decide to hook up to the grid instead of generating their own power. Or, put another way, the reasons a connected grid exists.
I’m far from an expert so it’s hard to really disprove your assumptions , but you make an awful lot of them. But I’d question costs of equipment and fuel. I’d question whether a diesel generator makes sense for a highly variable demand like DC fast charging. I’d question the emissions and efficiency. I’d question a lot.