Sure the amphibious IFVs and assault guns could cover the beach assault phase, I am certainly not hoping for a hot beach landing with LCACs loaded with tanks against a peer enemy and a sufficiently fortified beach, but I don't think amphibious IFVs and assault guns will be sufficient for bigger islands in the post-landing phase where the enemies have more area for defense in depth and the MBTs will be required for breakthroughs and anti-fortification work. In this phase the tanks can unload more safely.I think semi-amphibious IFVs and gun carriers have largely supplemented the role that MBTs would serve on a island hopping campaign, you need to remember that Sherman's weight about the same as a T-72, a full 20+ tons lighter than a type 99. I can certainly see type 15s being used, but type 99 esq heavily protected tanks seem too unwieldy to be practically used in islands, they simply can't be off shore deployed like IFVs can.
In fact islands might lack even the area for maneuver for these amphibious IFVs and assault guns which is another strike against their survivability and long term effectiveness in the campaign. Though I might be completely wrong and the next big thing in island hopping campaigns post-landing might be heavy IFVs like the VN-20 and BMPT terminator where they can carry out anti-fortification and basic anti-tank work with their ATGMs while having sufficient armour and APSes for line breakthroughs.