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Tony, among others reasons that cross world travel opens the door to villains cooperating with each other and possible multiplying the destruction they cause. it also means that other people can meddle in the affairs of other universes and cause headaches. Cap, with others thinks that if villains can cooperate, heroes can too and that means teams of heroes can combat bigger threats and minimize the damage. Also cutting off entire universes from each other means that if one is in trouble, nobody can help.
