In response to the OP:
The "YC2" thing is a vague title pending the defeat of Queen. Should that happen, well, it's not like Sanji won't take on a higher level opponent down the road. At this point in the series we probably have room enough for three major arcs, maybe even some kinda timeskip.
FS is great but not overpowered. Luffy still got smashed in one hit by Kaido and Katakuri is unlikely to ever compete with a Yonko. Sanji hastily learning it would be unlikely to compensate for the difference in speed.
BUT, when it comes to CoO, it's also very possible that Kizaru can not completely compensate for invisibility. We have not seen observation so good that it compensates for not being able to see a decently strong opponent. Maybe Fujitora but he'd be an exception given that he doesn't use his eyes at all and would then apply it at 100%. Unless you have the darkness fruit so far I think getting Kizaru to slow down is a much better option than actually dealing with his speed.
On top of that, his power being a DF means it doesn't fit the rules of science. Crocodile was weak against water on contact, Ener couldn't affect rubber, maybe Kizaru struggles with the light reflection.
As for the admirals, it's pretty likely that Akainu is just a stronger opponent overall. You might assume they're even because they used to have the same rank, but that doesn't have to be true. Ultimately he did in fact win against Kuzan, remember.
The "YC2" thing is a vague title pending the defeat of Queen. Should that happen, well, it's not like Sanji won't take on a higher level opponent down the road. At this point in the series we probably have room enough for three major arcs, maybe even some kinda timeskip.
FS is great but not overpowered. Luffy still got smashed in one hit by Kaido and Katakuri is unlikely to ever compete with a Yonko. Sanji hastily learning it would be unlikely to compensate for the difference in speed.
BUT, when it comes to CoO, it's also very possible that Kizaru can not completely compensate for invisibility. We have not seen observation so good that it compensates for not being able to see a decently strong opponent. Maybe Fujitora but he'd be an exception given that he doesn't use his eyes at all and would then apply it at 100%. Unless you have the darkness fruit so far I think getting Kizaru to slow down is a much better option than actually dealing with his speed.
On top of that, his power being a DF means it doesn't fit the rules of science. Crocodile was weak against water on contact, Ener couldn't affect rubber, maybe Kizaru struggles with the light reflection.
As for the admirals, it's pretty likely that Akainu is just a stronger opponent overall. You might assume they're even because they used to have the same rank, but that doesn't have to be true. Ultimately he did in fact win against Kuzan, remember.