We discover the island is a sort of purgatory or limbo gateway between life and death...

It wasn't, though. The Island was a real, actual place and everything on it happened as depicted. Basically, it's a Hellmouth with electromagnetism instead of magic.

The ending is something different, but no less heavy-handed. That's why everyone is there, whether they died on the Island or not. I quite enjoyed the last season except for the sideways crap and the ending. It just seemed like fanservice. Everyone gets a happy ending, guys! Maybe I'm made of stone, but I rolled my eyes.