I think you were just making the same argument that didn't get the point I was trying to make. I said I didn't give a care about what Paramount thinks. I won't restrict myself to canonical material. I don't care if the Sovereign is an accepted design. I accept it too. I don't like it a lot, but I accept it.
Now lets get back on topic, shall we?
The U.S.S. Sovereign is a ship of battle with all of what she's carrying, not a ship of peace, and Starfleet wants peace, right?
The most ideal ship for diplomatic/presidential purposes has to be the Galaxy-class Starship. Oh sure, the Sovereign-class Starship exists and it is one of the most advanced designs... Oh wait, the Prometheus-class is even more advanced. Now how do you account for forgetting it was canonical too? It's even more a ship of war than the slightly-less advanced Sovereign-class starship, which is slightly-more advanced than the Galaxy-class starship.
Be it Sovereign or Prometheus, a ship of battle is necessary to defend the president. This doesen't mean the president has to be in it. Frankly I think ships of battle are a bigger threat and a bigger target for the enemy to pick on. For the end of the 2360s it was the Galaxy-class Starship. For the beginning of the 2370s it was the Defiant and Sovereign warships. For the mid-2370s we have your Prometheus-class, and it was quite a sight indeed that the Tal Shiar wanted to capture the vessel for themselves and study it! If the president needs a ship of battle, he could beam aboard one and claim it "Starfleet One" for a while. Or he could order it to be an escort to defend the president. Have the warship as a distraction while the President cowers out to survive another day in his term.
The Galaxy-class starship is a ship of peace and exploration, and perfect for diplomatic relations. It is a class of starship most symbolic of the Federation, a government seeking peace and exploring to see what's out there. Also, how do you account for the absence of Sovereign-class ships during the Dominion War? There's only one hint and it was a remark said by Picard, and we don't know how many Sovereign-class ships have been made. It's a new class and 100 ships can't be easily spun out. If anything it should be harder to make than a Galaxy-class. And aside from practically the first of her class, the only known canonical Starship of said class was the Enterprise-E. Of course, having a presidential ship is non-canon, yes. But that doesen't mean we should ignore canon, as you frequently suggest. Also, there are about twelve, maybe thirteen Galaxy-class starships, a few of them with unknown names. One of them could be a presidential ship reserved for the President of the UFP to use for Diplomatic purposes, hypothetically speaking. It's also more practical to waste resources just for the President to have a ship slightly-more advanced than a Galaxy-class.
Quite frankly, I believe either way, the President would have a ship less powerful than another in the same class, and probably less advanced too. Should the president want defense, that's what escorts are for. You see important figures in cars guarded by police cars and motorcycles? Especially the President's car. They are his defense. And so should starships escorting the UFP's president, if necessary.
It just makes more sense to me to picture a Galaxy-class presidential ship guarded by a few powerful ships (I.E. Ambassador-class, or Akira, or what not), than just a loner armed-to-the-teeth warship painted to be a presidential ship.