That's by design though. The battle system in Final Fantasy X, XII, XIII, and XV are all completely different yet they are all in the same series. In Persona they added some more elements and guns but overall yeah it's the same core battle system. Like I said this isn't a series that takes risks. These are the same core games with some minor additions and a different cast of characters and story following similar themes.
Think about it these games aren't really that connected. The story lines are stand alone. They don't have to all be the same core game with the pieces shuffled.
In something like Trails of Cold Steel it would be completely weird if in Trails of Cold Steel II which stars the same cast of characters in the same world and continues the same story if you had a completely different battle system between sequels. In Persona they could completely revamp the battle system and still have the Persona aspect in some form like Final Fantasy's summons.
I guess the question becomes as this series gains mainstream popularity how long can they keep releasing the same core game before they catch shyt for it? I'm not a long time fan of this series. I only picked it up a bit after Persona 3 came out and got into it but I've even noticed that 3,4, and 5 are the same core game with some additions and new cast / stories. For many people Persona 4 was their first Persona and when they went back and played 3 they saw just how much things improved between 3 and 4 so it got a pass and now 5 is their first sequel they're playing. Personally I think 4 and 5 are almost too similar. 5 really follows 4's blueprint. while 3 and 4 were less similar than 4 and 5 are.