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The intermediate is non-aromatic arenium cation which is stabilized by resonance. Deprotonation of sigma complex is a fast process as it leads to much thermodynamically stable i.e., aromatic ring. Aromatization or deprotonation step is fast as Kinetic isotopic effect is not observed. Hence even a weak base will extract the proton from the substituted site which is sp3 hybridized. As for another nucleophile to attack the arenium cation, the positive charge is delocalised and deprotonation step is much faster.