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But SP would have no influence in rankings which means it would devalue

I wouldn't be in favour of 1A1V (including incentivizing the holding of Steem) if I didn't believe that it would increase the value of Steem.

Giving people a passive income (maybe part of the rewards-pool) simply for holding SP would stabilize the value of Steem.

Name 1 successful Proof-of-Stake blockchain ecosystem.

maybe Dash? arguably Pivx too... I mean granted they have problems too, but they are doing better than steem.