Keep in mind: the 'community' trusted Steemit Inc with their stake, without any enforcement in the chain itself

No this is false. There very much was enforcement, although I can understand how it might not be so apparent to newer or smaller stakeholders who were not a part of or well informed about Steem's history.

The stake remained transparent in the steemit account (and associated accounts) for the specific reason that it would then be subject to community oversight. Around a year or two ago when there was some consideration (albeit embryonic) about forking to exert more direct control over that stake due to lack of confidence in Steemit and Ned, Ned starting hiding it. This, in fact, made it more likely for a fork to occur, because failing to fork would eliminate on-chain governance over the ninja-mined stake. For that reason, Ned was convinced to stop hiding it and to leave it in a transparent powered-up stake, and at the same time stakeholders considering a fork backed away since the transparency and oversight could be retained without one. In some cases this was even a specific quid-pro-quo offered by some of the stakeholders and witnesses, though I personally never did such a thing.

In fact there has always been a degree of stakeholder control over that stake, and Justin will have to understand and respect this if he wants to gain support from Steem and its stakeholders.

I would imagine that this history and specific events (some of which documented on the blockchain and elsewhere) regarding the designated purpose, practices, and encumbrances of the ninja-mined stake should have been disclosed to Justin when negotiation for the purchase of Steemit Inc, since it certainly is relevant information about the company he was buying. If it was not that would be an issue for Ned and Justin to work out.

All that being said, I'm not 100% opposed to creating some sort of foundation or trust that could handle custody of the stake and enforce rules over its use, but frankly that seems a lot more complicated and time consuming to me than on-chain governance, to ultimately accomplish much the same thing (the stake used for the development of the ecosystem).