For legacy cars the likely answer is that people buy the $250 dongle. For new cars this will cost a few hundred bucks in parts and labor (or let's say 1-1.5% of the sales price) and it will be required surgery on your brand-new car.
Who is going to pay for this? If it's Tesla, then the smart move is to put CCS into cars right now (and as a side benefit, standardize their US and European models [edit: nevermind this bit, not the same standards].) If it's the customer, then it's an annoying non-trivial cost you'll have to bear. Yuck.
But it doesn't matter. The number of vehicles taken either way is negligible. In the old days, you had to change the miles/km speedometer to the other one when you took you car with you as you emigrated. Buying a charge adapter is easier.
Arguing about 120V is a red herring. Just about every US home has 240V service, most cars will charge at 240V not 120V.
Single phase vs three phase is the big reason for a difference between US and EU charger plugs though. Three phase is pretty uncommon in residential areas in the US but is common in many EU countries. Industrial customers in the US will often have three phase but it's pretty rare to see outside of that.
Who is going to pay for this? If it's Tesla, then the smart move is to put CCS into cars right now (and as a side benefit, standardize their US and European models [edit: nevermind this bit, not the same standards].) If it's the customer, then it's an annoying non-trivial cost you'll have to bear. Yuck.