Yeah, I agree with the assumption. Basically, off the top of my head, there are five ways to compete for users:

(i) Attractive content (i.e. content that attracts attention)
(ii) Superior blockchain technology
(iii) More/better dapps
(iv) Wider distribution of tokens
(v) More exchange coverage

Since this whole thing is a play on the attention economy, the first might be the most important.

If we don't have exclusive content, that places limits on the ability to compete on content. It's a balancing act, though, because we also need to attract and retain content creators in order to get more attractive content.

I think if I were the one making the decision, I'd allow voting for any content in order to attract users, but impose a cap on the voting percentage for non-exclusive content. For example:

Max vote = 100% for exclusive Steem content, Max = 50% for content that's also found elsewhere.

And in the comment, the curator would note that the post could have earned a higher upvote if the post had been exclusive to Steem.

As the months go by, depending on growth in numbers of active users, I'd start ratcheting down the max percentage for non-exclusive content, until it eventually reached zero.

Of course, the incentive for the curator would then be to vote 50% or less for everyone and not bother to find out if its unique or not, so it's hard to say. Hopefully, curators have enough investment in the value of Steem that they'd understand the importance of exclusive content. If that type of curation behavior emerged, it might have to revert quickly to a requirement for fully-exclusive content.