Encouragement:

Possibly the most important factor, like you said, is encouraging more curation by giving them 50% rewards. To some extent, potentially, that decentralizes the curation system. It gets more people involved in curating. That helps curators and it more importantly helps content creators.

Centralized Users

A single account may now only get 50% in rewards, in HF21, which is less than 75%, which we had before.

Mathematical Magic

Yet, ironically, mathematically, curation is like a funnel. The single user is a central point. Instead of increase the smaller end of the funnel (posting rewards), theoretically, expanding the other end of the funnel (curation) has the potential towards expansion. I have my fingers crossed for this new hard fork.

Content Creators

Some might argue that content creators, to some extent, might be like employers. Imagine allowing employers 50% profits. I like that. Some might say that's too much. But before HF21, it was at 75%. So, this might be a step in the right direction. And other people argue that it should remain at 75% or more. I understand their concerns. But mathematically, the curators are like the employees. They upvote the content that is provided by content creators (employers). If you give a thousand curators more money, then they might curate more. And 50% of that goes to the employer. More curation means more votes. And more votes means more money for the central users. So, in theory, this will improve the Steem system. Mathematically, it's easy to understand the funnel system. I hope this second pool, the downvoting system, helps as well.