No-one wants to punish the UK.
But once we're out, and need the unanimous agreement of every member state for a deal (rather than qualified majority now), then Spain may raise Gibraltar, France will not agree to anything that reinforces London's financial clout, carmakers will see no reason to revive the dying UK car industry at the expense of their own, and other nations may have their particular issue. Punishment isn't the problem, it's an inevitable lack of helping the UK given that we've chosen to damage the EU as well as ourselves, and we've poisoned the well of goodwill