Equipment was omitted from the RAN ships, but installing it would not require any major structural alterations. The bulk heads are all the same, the usage of those compartments is different. If as the RAN said themselves, they couldn't afford the extra cost of redesigning the bow to omit the ski jump, why would you think they could afford a much more radical redesign of the interior compartments and layout to preclude Fixed wing aircraft? Those compartments originally intended to house support facilities for jets were simply reallocated to other purposes. They can be reallocated again. The idea that these ships were altered so as never to be able to operate the F-35B is a story put out by the RAN to avoid opposition from the RAAF to the ships.
I agree they aren't ideal for operating Lightnings despite being designed for the job. My main opposition to them would be they have far too small a flight deck for their size. For a 27,000tonne ship they offer hardly any more deck space than the 12,000tonne Thai carrier Chakri Narubet, The Spanish designers can hardly claim they weren't intended to be carriers because they absolutely were! It was an integral part of the design brief and they could have done a much better job in the first place.
View attachment 49399 View attachment 49400 RTN Chakri Narubet, Half the size of the RAN LHDs, same size flight deck and hangar.
View attachment 49401 Now if they'd done this from the start, they'd have much more useful ships all round.
View attachment 49402 But as they are, they still pack a lot of untapped potential.