Redirecting from SP interest ends up increasing overall effective inflation. In reality, the only effect of SP interest is to allocate the other inflation costs between powered-up and non-powered-up investors.

It's kind of unfortunate that SP interest aka vesting rewards constantly ends up being described as a portion of inflation (including in the steemitblog post pie charts) because its function is really quite differently from the others.

I also agree with gtg that the content pool and SPS pool are natural equivalents. They both involve making a tranasction to the blockchain asking stakeholders to vote to pay you. The only difference is the particulars of the voting and payout rules. You could actually look at SPS proposals as being a type of post.