Hi my friend,
I've been testing the tron blockchain a bit and I believe that you are right to point out that steem can't work on top of tron. The main reason is that the transaction costs on tron are simply to high for such a social network to function properly. Each trigger of a smartcontract costs you trx or requires frozen trx. Steem would have to be superposed as a smart contract and most users could simply not afford to pay to vote, to comment or to interact. I believe from a technical aspect, tron would be much better off if the whole social media went on working over the steem blockchain.

When I look at the companies that Justin Sun has bought like Dlive or bittorrent, he hasn't changed their business model and their core concept. I don't know why he would do differently with steemit. What I believe however is that there will be an interchangeability of tokens on both blockchains. Like you can have splinterlands DEC token on the steem blockchain and on the tron blockchain. It's possible that you will have to go through tron to buy and sell steem tokens to access the markets.