So a few months ago I made a thread on how ArenaNet will release content and I think I was right.
They start off with living story and at the finale of the living story an expansion is released to fight an elder dragon. Once Modremoth is killed by us a new season of the living story will release and at the end of that season finale there will be another expansion to fight an elder dragon, and so it will keep going.
It’s pretty obvious now.