I am just wondering, did you take in account that users will try to get the set of witnesses that maximise their profit?

I did, yes. :) There's a bit of set theory in the proof that I glossed over, regarding the convergent set basically being the quickest to the bottom. But a given subset converges to the same isomorphic maximum set (of all 100%), and which subset wins is actually arbitrary for the proof. They all work.

On a more game theory side, every witness giving dividend seems like a natural equilibrium.

That's right, that's kind of the point of the proof. It's equivalent to lowering the block reward when done in unison. But the block reward is there for a reason, no?

Looking forward to more comments from ya! :)