[PunchyHamster]

well aside from being effectively 20 times more expensive over entire operation...

DC switches (as in, just a power switch) are vastly more expensive because while in AC you have 100 breaks in current a second, DC is constant so it is far harder to break. So even if you had device that could use both (not hard with SMPS, they have rectification as first step), it's still essentially " replace everything".

[VgsIndustries]

The protections side is a big problem - most HVDC has circuit breakers operating on the HVAC sides of the link so going to full DC transmission presumably wouldn't eliminate that equipment.