Yes, SP designed to reward holders with influence, and for their investment to appreciate over time. But this appreciation comes at the cost of the other purpose of Steem which you have not mentioned - distributing rewards to other members, not yourself. Every token that someone allocates to themselves is necessarily not allocated socially. Steem is not simply an interest deposit where you can post any old thing and self vote to get your "dues".

Increasing the value of the Steem token is not a simple cause and effect, it's complex. There are a lot of people bringing up the concern that self voting is in fact scammy behavior and this might actually go against adoption and the value of the coin.

Nobody is proposing to deny SP holders influence, it's pretty much gospel here. But letting their influence increase too much is not the goal either.