I think it's rational optimism. ER dwarfs every soulsborne release to date, and I think its DLC will scale in a similar ratio. A lot of the fundamentals to world building are in place, and the scope of the world map lends itself to multiple expansions, not just one. You don't hire George R. R. Martin to write a backstory and not attempt to milk it. This DLC and hopefully successive expansions will be substantial.