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It would be great if they did, but it's not 100% necessary. It would be possible to build this as a "community plugin" for Steem blockchain.

Witnesses ultimately control what software gets deployed.

But at least the site steemit.com is 100% property of Steemit inc afaik, so you can't force them to cooperate considering what steemit.com visitors will be able to see. Right now there's quite a few blockchain functions not supported by steemit.com ( like acc creation, convertion and so on ) and there's no reason to expect this attitude to change in future.

True, but steemit.com is not the only interface to Steem blockchain, there are great options out there - steempeak, steeve, busy

True, and from none of that you can expect any cooperation for granted. That's why first thing I mentioned that you probably have to consider launching your own client prior to any other plans.