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Yeah, technically that's true, we could do it without them. However there's performance issues with increasing the payout duration (it's more posts that witnesses have to track) and we'd have to have someone change the code, testing, and getting it approved as a pull request.

Yep that makes sense.

On a slightly related note, do you how this would work with SMTs, in theory? Could you make your own payout rules and rewards rules, or would they all essentially function like steem?

It could - but it won't be possible until the main steem change allows for changes within itself too. That stuff right now is just all hardcoded right into the software so nothing can change it (not even SMTs, yet). It'd be awesome if you could adjust that stuff though.