This assumes Italy wants Munich enough to commit two units long term to a plan that still doesn't gain them the city (which would better be interpreted as having some reason to want to bog you down). If they wanted it that much, they could just move Tyr -> Boh and Ven -> Tyr next turn (this being in the counterfactual you assumed where they would have one build next year). So if Italy really wants to set up an annoying stalemate around Munich, they can do so whether or not you defend it this turn.