What is the thought process behind this take
Haki progression.
Remind me how many times Sanji was focussed CoA or even CoC type Haki.
CoO came as passive use only.
Couldn't even fight with Vergo either.
He isn't shown to have Hardening as of now properly.
Haki progression wise FS makes more sense than CoA or CoC upgrade.
Luffy and Zoro extensively used CoA , Ryou and when capped out , Luffy learnt AdCoC and Zoro unconsciously tapped into AdCoC.
Now guy with nearly zero CoA feats, out of random unlocks Ryou , then CoC , AdCoC. Make it make sense.
Sanji title was " Warrior of Science " he will be more or less linked with scientific Development only.