I get your point about 3 Commando Brigade, as it appears only (the undermanned) Forty and Four Five Commandos still have a 'traditional role' to any extent, Four Two has a dog's breakfast of roles:
high threat Maritime Security Operations (MSO), Maritime Interdiction Operations (MIOps), Joint Personnel Recovery (JPR) and Support and Influence Operations (S&I). 42 Commando is a VHR commando force capable of delivering special operations with a specific expertise in maritime operations.
(RN website).
Mind you none of them seem to have a dedicated role, 40 & 45:
rapid reaction, amphibious warfare, Arctic warfare, mountain warfare, expeditionary warfare, humanitarian support, and disaster relief.
To my (old fashioned) way of thinking an LPD is still an essential piece of kit for landing and moving a RM force in support of the NATO Northern Flank as they and their kit and supporting arms may need to go somewhere away from landing sites or in conditions that preclude delivery by air in sufficient numbers. It appears to me, but I may be wrong, that The Royals are still a major part of the UK's purple commitment to the High North, assuming my reading of this document (
The UK’s Defence Contribution in the High North) published under Ben Wallace in March 2022 is correct and the policy still valid. In such a case the LPD Command facilities also come in to play, not something I believe the current RFAs can provide, and we can't afford (or man) a dedicated LCC style dedicated C4 ship.