I think the name Steem will stay with whatever chain has the most value and usage

I think this is a very naive view of how things work. My guess is that Justin is already very close with the key people at the major exchanges and has most likely already discussed this with them. There is no way he would make this acquisition, do the work of moving everything to Tron, and then have his token not be "STEEM".

The original "STEEM" will have to apply for a new listing on exchanges (which will be difficult since all of the exchange relationships left with steemit, inc) as Steem Classic or something like that.