It's not 1/3, it is about 150% of the currently active voting stake, meaning 71% of all actively voting stake, were it to vote.

Perhaps some more stake might vote but there will always be some held by dormant accounts/passive investors who literally aren't paying attention or aware of any governance decisions, by dead people, by accounts with lost keys, by custodians with a policy of not voting, etc.

Claiming that 29% having much of a say is a push for 'decentralization' is a stretch IMO.

The core problem is simply that the stake is too centralized, was too centralized to begin with, and people were led to accept this situation by a bunch of empty promises, misleading and outright false statements, and a misplaced sense of trust in the founder (even, somewhat inexplicably, after the other founder demonstrated one reason why one should not trust in a founder).

That being said, I've noted elsewhere that I think splitting block production and governance (as was the case in Bitshares before this was combined in Steem with no explanation beyond "simplification") might be best, and even your exact proposal might be good, as long as people recognize it for the tradeoff that it is.