Then you still end up with some players who are big and get ahead and some players who are small and stay behind. The magic "We all expand at the same rate and then the borders move constantly because everybody is losing cities left and right!"-gameplay just doesn't happen - at least it hasn't happened in past Civ-games. If they found a way to fix the fact that large empires just get ahead so quickly that they can easily stabilize and actually have to spend a considerably time defending themselves, then that may be a different issue (Although at that point it's questionable whether it's still the game most Civ players want).
In a scenario with controlled expansion you still bump into each other in the midgame, and at that point Civ V did a horrible job encouraging warfare, so I hope the less strict system of Civ VI fixes that and actually brings the "Conquer to keep up with others who conquer"-gameplay.
Civ VI does a little bit of forcing careful expansion by making Cities not being able to defend themselves on their own, but I highly doubt that will be enough to force the gameplay we're looking for to make such a scenario realistic. Especially given that the AI would be a LOT worse at it than human players.
I think you kind of got my bigger point which, to be fair, I did not really disclose earlier.
The main point is that we can't criticise civ 6 based on our experiences of civ 5. Which is why I don't think ICS is even the right term. ICS had to do with being able to found cities that had enough happiness to counter the cost of founding a new city. That won't be a problem in civ 6.
The opportunity cost of building your 10th settler, compared to whatever else you could do with the same production will slow down the expansion, at least in the early parts of the game. But the more production the cities have, the less significant the opportunity cost will be. REX will still be a viable strategy but it will have a cost associated to it; you won't be able to do something else.
I'm sure expansion will be encouraged but there will be different strategies on when, where and how to expand.
Having said all that, I think warfare will evolve quite alot in the first add-on, just as in the previous iterations. So it might not be up the scratch in vanilla. Hopefully I'm wrong though.
E: I didn't really enjoy how the mechanic worked in civ 5. And I don't know whether civ 6 will do it better. But to criticise the game a priori, based on our experiences of civ 5 isn't really the way forward either. Also, having watched quite a few LP'sof civ 6, I've not seen a big problem regarding REX.