You know Mario and Rabbids Sparks of Hope?
(Bear in mind, I have never played this game.) When Rosalina turns bad, she takes control of Bowser’s minions and all of mushroom kingdom. So it’s at this point, Bowser finally realizes: “If I can’t beat Mario, join Mario!” Is that correct? He realizes now it’s best to just fight for Peach and Mario instead of force Peach to marry him like in Super Mario Odyssey.
Mario straight up kills bowser toward the end of New Super Mario Bros before being revived by a spell from Bowser Jr.
That’s where “Dry Bowser” comes from