Steem 100% migrates to Tron and our blockchain is left derelict.

This is what I think will be happening.

I don't see a reality of how this can happen...You can't just plop all the Steem code on Tron and expect it to work.

You can't just "plop" it, but you can build it. They said very clearly in the announcement that:

TRON and Steemit’s development teams will immediately begin working together to bring Steemit and other Steem blockchain based DApps to TRON blockchain.

So they make a new token on Tron called "STEEM", and they work with the Steem core developers who now work for them to rebuild the Steem posting and voting system as smart contracts on the Tron chain.

It makes a lot more sense for this whole "token migration" business to be a two-way street that goes in both directions permanently.

No, it doesn't. Justin Sun wants to take Steem and make it part of Tron.