Is Hive more decentralized than Steem? Yes.

Are you sure about that? I am not so sure. Both are pretty much centralized around the whales. They control (own) the content visibility, and they also decide who get the most part of the rewards. Like you wrote, it should not be any whales on DPOS.

Will the whales sell their stake to help make Hive more decentralized?

I think that it would be foolish to assume "maybe", let alone "yes" as an answer to this question. They took control and richness, and they do whatever is needed to keep and grow it. Power corrupts most people, and the rich almost always want to be more rich.

But to be honest, if I would have 150 000 Hive power, I would not sell it either. I would use it to curate, and I would also invest it.