In practice, RSUs already work that way: you're taxed on RSU delivery, which in the case of a public company is almost always the same as the vest date. Usually the company will sell a portion of the RSUs vested to do tax withholding on the spot, and then you're immediately free to hold or sell the remainder.
I the case of a private company, RSU vesting/delivery means you owe taxes but (usually) can't sell them to pay for it, which is (also) no different from today.
I the case of a private company, RSU vesting/delivery means you owe taxes but (usually) can't sell them to pay for it, which is (also) no different from today.