This has been ongoing for some time and easyJet only make the service available to 30 pax per flight, not more. You effectively get the chance to pay extra for a boarding card in the 1-30 group called for boarding first.
Now if they only took the extra six seats out of the 319s, reduced to 150 and put the first five rows at higher 34-35 inch seat pitch, they could charge even more for this product and cut costs by removing the fourth cabin crew member from the flight. I'm sure the revenue from selling these seats on every flight would outweigh the revenue loss from the 10-15% of A319 flights where the last six seats are occupied.