Is there any chance companies could fairly easily adapt to this? For instance, grant 30% more equity per year than they otherwise would and buy back that 30% as it vests to cover taxes.
Edit: Now that I think about it those numbers don't work because it's an extra cost for the company to buy back your stock. So you end up with fewer shares with this scheme, but the company has also effectively paid off your "golden handcuffs". So in some cases it should even out in the end (you don't pay those taxes in a liquidity event) and in other cases it's actually better for the employee (you can afford to keep more of your equity if you leave before a liquidity event).
Edit: Now that I think about it those numbers don't work because it's an extra cost for the company to buy back your stock. So you end up with fewer shares with this scheme, but the company has also effectively paid off your "golden handcuffs". So in some cases it should even out in the end (you don't pay those taxes in a liquidity event) and in other cases it's actually better for the employee (you can afford to keep more of your equity if you leave before a liquidity event).