The only thing that quarantine would potentially do is isolate a new variant coming in.
How many variants have we had so far? 5 I think? Each of them covered by existing vaccinations. WHO estimates suggest that over 2 billion people have been “infected”, so roughly a quarter of the worlds population. Now let’s extrapolate that and workout the risk of an international arrival importing a new and undiscovered “variant”.
We’d be better off closing the borders to prevent the importation of terrorists, as that would be a significantly higher risk.