Replacing Fast Battleships with Large Cruisers to provide air cover for carriers has the greatest advantage of adding a Main Air Defense Ship without increasing expenditures.
In practice, it had proven that a single Fast Battleship could not provide full defensive air cover for a carrier.
The crux of the issue was that, in the midst of a high-intensity naval and aerial battle, no one could accurately predict the direction of incoming enemy aircraft, which meant that Anti-Air Warships couldn't always be in the most suitable position.
In several naval battles fought after the Newland Federation entered the war, there were similar occurrences where, at times of enemy aircraft attacks, Anti-Air Warships were not in the best position and thus unable to provide complete cover. Sometimes, they couldn't even offer any protection to carriers.
