The chassis are virtually the same, the new MBP is just a speed bump. They could design new keyboard to be backwards compatible with the old mounting points and connectors and simply replace old keyboards with the new ones. There is no new ground breaking features between old and new keyboards apart from that membrane. I think they just hope that after replacing old keyboard with the same ones will be enough to push those units trough warranty period and if keyboard fails after warranty then a lot of people will be pushed towards buying whole new unit. The second reason is that during the recall Apple is able to push out old stock of keyboards, if they would use new design keyboards the old stock would have to go to recycling witch is costly.
