The storms had been forecasted. Last year Augusta moved up their final round when bad weather was in the forecast, no reason the PGA couldn't have done the same. Even without the bad weather, why schedule a tournament to end at 7-7:15 PM when there is the possibility of a three-hole playoff? Regular events end at 6 and they only go to sudden death.
Not meaning to imply that they should have played through the storms, but that the final group should have been off by 2PM, and moved up further if possible once the storms were forecast. Mickelson had already commented on the lack of light at the end of Saturday's round.