[spoiler]You're missing the point, narrative and theme wise. The game has been telling you not to take the easy way out and always search for the Truth, even as early as the Rise arch you start understanding that finding the Truth isn't going to be easy, we knew that Stalker wasn't the murderer, but the people in Persona land didn't at the time even if it was obvious because they only see what's in front of them, they don't know they're in a story and the game isn't even half over.
People are satisfied in finding what they 'think' is the Truth, we're blinded by preconceptions, perspectives, and bias so we will take a False Truth to satisfy ourselves. In the Naoto arch Naoto herself is frustrated because she knows something is up and Mitsuo wasn't the real murderer, the facts and evidence just didn't add up at all. But the police were eager to put the idea of that behind them simply because they want it to end and to make the public feel safe again.
It's why the You and the Team thought Nametame was the murderer, even if something wasn't right you wanted him to be the murderer because you were mad that he caused Nanako's death, they were blinded by their rage, and they wanted to 'avenge' her even though killing him isn't going to do anything. Killing someone who killed someone else wouldn't bring the person killed by the person you killed back, you'll just have 2 people killed. What's more Revenge and Justice isn't the same thing, Nametame wouldn't pay for his crimes, he would die because You and the Team didn't care if he was the murderer or not, you wanted to justify in your mind that he was the murderer so you could justify killing him and have your revenge. Only by figuring out something wasn't right and thinking clearly would you realize that Nametame was just another victim himself, a patsy tricked by someone else into believing he was doing good, the person who really murdered Saki and Mayumi Yamano. And that person was Adachi.
All this lead to you having to figure out that Adachi wasn't the real source of everything, he just committed the first 2 murders, all your questions about the Midnight Channel and all that wasn't really clearly answered. You weren't supposed to think there's another criminal out there, but something else was pulling the strings that led up to these events, the means to even make them possible, like a higher power just gave someone the potential to do what they want and they did what they wanted with it while they sat back and observed.
Let me put it to you this way, you know that for the Team in order to get a Persona they had to accept their flaws, Shadows, as part of them and not deny them. When they did the Shadow becomes a Persona and subsequently they get the ability to enter the TV on their own free will. Cool right.......except You didn't do all that, you pretty much somehow magically got it and you've always been able to enter the TV, unlike the rest of the team. Further more why does Adachi have a Persona, he sure as hell didn't accept his flaws, he denied them and blamed everyone else for his short comings and misfortunes. You know who else did that? Mitsuo. And you know what happened when Mitsuo denied that he was an empty loser? The Shadow disappeared and he didn't get a Persona, so he became a lost cause. What's more how the hell was Nametame able to enter the TV, he only entered it when he kidnapped Nanako and was desperate to get away when the jig was up. He only saw what a crap sack world the TV world was when during that time. But you need a Persona to enter the TV and he sure as hell didn't have a Persona, he was possessed by random Shadows even.
Then you have Ameno-Sagiri who came out of nowhere and raised even more flags, first he said he didn't count on the potential of people facing themselves and gaining Personas even though Adachi had a Persona and he claimed that he was following his desires, so how was he not aware of that. Then when it dies, it says it will be back when Mankind wishes it to be back and then be enveloped by fog.
Keep in mind that Marie was not in the Original version of the game, so that's where the Team thinks it ends and they celebrate that they don't have to deal with all that crap ever again and You start getting ready to go home.
But then you have all those questions in your mind about You and Adachi's Persona, You, Adachi's, and Nametame's ability to enter the TV, what the Midnight Channel really is and where it came from, and what the hell did that funky demonic disco ball fog machine of doom mean when he said 'I'll be back'[/Terminator]
'Wait a minute none of this makes-OH CRAP!'
IT'S...NOT....OVER.
You may have found out how the murders came about but you never figured out how You, Adachi, and Nametame were able to do all this crap without going through what everyone else on the Team did Which means.....it could happen again.....when you're not around......it will happen again, and you need to stop it because the next generation might not be able to. You need to find who gave You, Adachi, and Nametame that potential in the first place or else the Fog will come back.
If you accepted that Adachi was behind everything without questioning how he was able to do that, you stopped trying to find the Truth and accepted yet another False Truth, thus dooming Inaba to eventually repeat these events again. But since you did ask all those questions you were able to find the Truth and prevent a future catastrophe.[/spoiler]