I suppose there could be groups of people like @steemcleaners that will check if the content was already posted somewhere else, and take the necessary actions if that were the case.
In Steem for example, we could start using a new tag like "steemexclusive" to let the reader know that content is only available in Steem.
And yes, the whales circlejerking can be a risk if it also happens in other platforms, the same goes with the people who "predict" prices.
But as you said, we don't know how the other platforms are going to work as of yet.
PS: are you planning on using any of them? or you will keep using just steem?