Marika isn't marking Ranni as the winner, she likely knew Ranni would straight up be cut from her body and thus her fate, Ranni cannot win the whole shebang because of this, she is choosing Ranni because Ranni will not chose to stop the War or solidify everything.
and Did Marika set into motion freedom? do you really think our PC will stay the Elden Lord? the entire speil from Gideon about Marika's intent stats taht she knows a man can never kill a God, aka HER, and by this he means no one can turn the system off, even turning her into a frozen Statue, she's still "there" and all her reality warping still in effect, all you can do is steer the ship or straight up destroy the Universe in the Frenzy Flame ending, which she would be fine with.
Only Ranni seemingly, can send her out of this world, since she's an Empyrean and you basically grant her the might of the Elden Ring in that ending, this doesn't really end the conflict, after you leave, people will fight over the rubble, Marika simply wants everyone to fight over ruling, forever.