I think Oda is using that not to simply make Fujitora switch sides, but to make his fight against a Strawhat (Zoro most likely) more "emotionally driven" (which kinda fits Fujitora's character), and consequently epic.
Zoro vs King had that feeling, it had a few flashbacks/compliments about Zoro and his peculiar way of seeing Luffy as his Captain, just like Fujitora also sees Luffy in a very peculiar way. So as i said, i don't think this is here simply to prevent Fujitora from being an enemy, but to give his fight against Zoro much more content, which would also add a lot to Fujitora as a character on every single way.
For me, the way Oda portrayals Fujitora and Ryokugyu is almost enough to make me think they are indeed going to be Zoro and Sanji's future opponents.
Although i get the Zoro vs Kizaru appeal, Kizaru always clashed with Right Hands, he can create a Light Saber to fight, and he gives the Akainu's Right Hand vibes (im not so sure about that tho).
But that could actually mean something deeper. Oda took the time to introduce Fujitora much earlier, instead of offscreening him until he got to do some real shit, so someone could argue that the hype around him isn't that big anymore.
Oda also placed him in situations where some people can come up with their agendas, although im sure most of the Fujitora downplay is dumb as hell and makes no sense.
What Oda usually does with Zoro and Sanji's opponents is that he introduces them very late into the story and we take quite a long time to see them on some real action, in order to build more hype when their 1vs1 comes.
That could be the reason Oda is offscreening Ryokugyu so much and building mystery around him, the first time he fought also happened offscreen and he wasn't even shown in the manga afterwards, but Fujitora was.
It's like that event was completely irrelevant to Ryokugyu.
So based on that, i could see Fujitora switching sides, Ryokugyu would refuse allying with Fujitora so the Admiral Trio would end up being Akainu, Kizaru and Ryokugyu. That decision alone would automatically add a lot to Ryokugyu's character by itself, even if he isn't actually being properly shown in the manga (its a good claim imo)
Back in W7/EL, Lucci and Kaku were already there, Jabra came much later into the arc, but Lucci and Kaku already knew him briefly, and he indeed fought Sanji.
So it's not weird to believe something similar could happen again with their next WG foes, especially on how Oda loves to stick with all kinds of callbacks for the Monster Trio.