The 5 year mission was TOS. In TNG the enterprise barely leaves federation space, it's not on a deep space exploration mission. Even in TOS there's plenty of times where they go back to earth, or are called to a colony or starbase. They never really seem all thst far from home. And yeah, you should probably have some degree of armaments on your exploration vessels, they use em to get rid of debris or to technobabble away anomalies by shifting the phase variance, so some degree is good. But they also really should have been building stuff like The Defiant way earlier for home defense. There's a lot of dangerous assholes out in star trek space.