Justin Sun is a nobody to noncoiners. To those with cryptocoins he is mostly considered a successful, but scummy marketer.
It will boost price if true, but it won't actually make steem any better unless it attracts developers and development. Looking at Tron's other projects I doubt this could be a long term good.
If he buys SteemINCs stake, a fork to remove ninja mined Steem should happen. Play both chains or sell one and stck with the other.
Imagine in the event of a sale, a fork where all the ninja mined steem is burned or moved to the SPS instead of sold to Tron Inc.
That can not happen unfortunately, knowing people of Tron, they would never buy Steem without that stake included...
In the best case : Steem and Steemit stay as it is and we will all get benefit of that new partnership. So the Steemit team can continue their work and deliver the new promised features and improve the chain with less difficulties.
In the worst case : they will use the stake to put their own witnesses on Steem, make their own promotion on Steemit, and so until Steem, Steemit and we falls.
I cannot see the worse case you mentioned being profitable or wise for even tron.