Eh the retcon was needed because it let them expand beyond 14th century Europe/China and make it a "timelost island". Want Roman Empire and Vikings? you got it!
Yeah this is totally wrong, you can totally add all those things, in fact you can expand on them and increase the mystery, by keeping the original lore in tact.
We were all age of sail conquistadors and explorers in search of the new world and we get sucked into a vortex/whatever the fuck and end up on this cursed magic zombie island thing. We're all starting at now and building up this beach head.
As you explore more, new expansions come out, the island gets bigger, we're able to explore deeper into the jungle, find more ruins, and we find even weirder ancient ruins from explorers before us. ruins of Vikings who got shipwrecked here. Ruins of romans, egyptians, shit is this atlantis? you can expand it as much as you want every expansion because the island is this big magic mystery and we don't know shit about it other than its fucking cursed.
But they changed all that. We're all immortals, we've always lived here, one big happy family. The settlements have always been here, always occupied, we all live in harmony with one another. There was never any survival element or mystery to anything, we wash ashore when our boat wrecks and welcome to disneyland, watch your step. the Egyptian will take your coat and the Viking will validate your parking.
by retconning that all these other different groups were always there on the island, everyone is immortal so there was never any gaps the islands have always been inhabited then there was never any ruins or mystery, the entire world is known and mapped. everything is just built upon what came before. We're no longer lost explorers arriving at a mysterious Island we're the 50th group to go through the ride today at disneyworld.