If one wheel fails, you are still ahead in the game. Under those circumstances, that wheel gets all of its braking from the driver's pedal effort.

If the wheel locks up, the driver must then lower the braking effort, and normal, w/o ABS.

But if it does not lock up, the others are stiil being 'enhanced.'

Plus, a one-wheel lockup still might not be so bad. There would be side-load applied to the other wheels, inspiring possible wheel-slip, but the ABS on thos wheels would restrain the wheel lockup that could ensue.

Brian