I agree that the Harrier should not have been taken out of service; it was the most flexible role aircraft we had. It was also getting older and a bit expensive to maintain. They were also starting to become technologically dated against newer weapons systems/aircraft.
As you say the only available replacement for the Harrier is the F35B which is a great shame since Harrier was developed at the Hawker/Dunsfold sites, which are now closed, with the loss of a huge amount of expertise.
Although a conventional fixed wing aircraft could operate from carriers, the current design of those being built does not allow for them to do so and modification would be very expensive. Perhaps if a new generation of carriers is considered they may (at a cost) have both 'cat & trap' and 'ski jump' options incorporated in their design or perhaps be of two types - this will be a question for the politicians an MoD at the time.