Actually, Rodan never was a titular antagonist for Godzilla. He fought Godzilla fairly evenly in Ghidorah’s premiere movie, and then got absolutely destroyed in Heisei (Godzilla vs Mechagodzilla 2) and Millennium (Final Wars).
Ghidorah, for all his roles in the Godzilla franchise, has only been the sole monster for Godzilla to fight once, in Heisei. In Ghidorah the Three Headed Monster it was Godzilla, Mothra and Rodan vs Ghidorah, Invasion of Astro Monster was Godzilla and Rodan vs Ghidorah, Godzilla vs Gigan featured Ghidorah alongside Gigan, then he got gang attacked in Destroy All Monsters. 1v1’d Godzilla in Godzilla vs King Ghidorah, fought him in GMK and then Kaiser Ghidorah fought him in Final Wars.
I know it’s nitpicking but I wanted to point it out.
Edit: Something else I thought of is that giving each monster in King of the Monsters their own fight with Godzilla would cause issues. A Godzilla vs Mothra and Godzilla vs Rodan for Monsterverse would really only serve as filler for what it’s actually leading up to, since we’d know that both of those theoretical films would pretty much have to follow the same plot so that neither Mothra or Rodan die before its time to fight Ghidorah. I can understand the concern, because BvS and Justice League sucked so much, but that’s because for a superhero universe you would want the individual movies for each character, so you could see them grow and develop, be introduced to their personalities before getting into the meat of some big event. You wouldn’t want that for monsters. You’d want to see some awesome action and some awesome kills. You wouldn’t get that with an individual vs Mothra or vs Rodan film. It is both financially and artistically a better choice to go straight to the Ghidorah brawl where a meaningful fight and likely some meaningful kaiju deaths are going to be had. Even if the film is bad, it’s not going to be bad because Rodan and Mothra didn’t get their own films.
Also, I know I already said this, but the Godzilla franchise has already done what MCU has done. The Showa series, the first of all the Godzilla series, basically had its ‘Avengers’ style crossover in Destroy All Monsters. MCU definitely popularized it, but if any franchise has its roots in bringing monsters together fast, it’s Godzilla.