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Or the detriment in the long run
Some core principles benefit citizens. The UK pissing on other EU citizens for example is not something that necessary will benefit other EU citizens.
If the EU dilutes freedom of movement, which is already reliant on having work / money / family reasons, other EU countries may follow to attempt to lock in their citizens or screw over EU state immigrants.
Not that this is so great as everyone will hurt but there's a risk of fewer rights / protection with concessions.
Playing devils advocate - because it might be to the overall benefit of its actual citizens.