This is the F-35 equivalent of having left the space on the QE2 carriers to fit catapult equipment, which could be then be fitted at reasonable cost when required, right?
This one actually gets my goat as it's often used as a bit of a stick to beat the ACA with without properly thinking about whats involved.
The carriers were designed big enough to take catapults and traps without having to rebuild an entirely new vessel, doesn't mean that it wouldn't be an invasive and long process. However when the carriers were designed EMAL's wasn't a known locked down design which we (the UK) would have had access to. So any 'reserved' space would be based on assumptions and previous experience my understanding is that the original design reserved the spaces needed for steam cats and traps so it's not totally suprising that the possibility of fitting an unknown design with a totally different footprint and form factor (EMALS is only going in new build US carriers, AAG will though) into the ships has resulted in a big design replan hence massive costs and risks and through life support costs which I'll bet that big figure bandied about includes, not just the cost of the 'parts'.
IF it had been a properly serious goer when the true number of design changes needed for EMALS appeared then they would have gone for steam cats and traps but if you read through freedom of information requests relating to that perfectly reasonable question the MOD were dead set against installing steam boilers in the ships thus steam cats and traps was always a non starter.