Full foiling way faster. On the moth the F16 cats in Singapore can't keep up. Passed them to leeward going upwind, and downwind going faster and lower. And the moth only has an 8 sq m sail...

And as for costs of the foils, on the Bladerider and Mach2 the foils and the foil set up contribute to half the cost of the boat (retail is US$14k-$17k). Adding that cost onto the current price of the A would mark things out of reach for an already successful class. Plus the current A class masts couldn't withstand more than a couple of crashes from foil ventilation as you certainly hit the piss hard when crashing.