Why amphibious lift is part of blue-water power

What would the Royal Navy gain by abandoning amphibious capability and concentrating only on blue-water power projection?

Very little, because amphibious warfare is itself a blue-water capability and Britain is already weaker after reducing the ships needed to land troops from the sea.

Amphibious lift is part of projecting force across an ocean because troops and equipment must reach the shore as well as aircraft reaching the theatre. The force concept described here combined two Queen Elizabeth-class carriers with three LHDs able to operate F-35Bs. Five flight decks and three well decks were intended to leave at least two flight decks and one well deck available. That availability explains the appeal of a common short-take-off and vertical-landing aircraft across different ships. The French LHD option considered here lacked the required F-35B compatibility, with lift size and deck construction identified as obstacles. The criticism is that failing to acquire the accompanying LHDs weakened the intended force and limited seaborne options in the Greenland example.