There's no intrinsic reason Uber's take is fixed at 20% - maybe their business can only viably employ humans (with all of their attendant needs like health insurance) with a smaller take.
It would be likely for drivers pay to be cut in half, if Uber had to pay them as employees (just based on comparable differences between paying contractors and employees in other fields). Their taxes would also go up since they could no longer deduct the cost of their car and other expenses from their income.
Even if you pared Uber's take back to 10%, or 0%, it wouldn't cover the costs of making them employees. And it's the drivers that would take the hit.
Uber provides a tremendous lead-gen service for drivers. Everyone here who's tried to run a business knows that finding customers is the central challenge of a business. Uber completely handles that challenge for drivers, for a risk-free 20%. That's a pretty good deal.
