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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.
Put another way, why would they change their core principles, diluting them, for a member who is leaving?
If we were staying and (say) Spain was leaving, what would our press and our parliament be saying about corrupting the single market to allow Spain to benefit from it once they'd left?