Why arent the other vinsmoke coo beasts then ?
They are happy to rely on the technology, Sanji is not and is only doing so begrudgingly.
Also they aren't as fast as Sanji is in base, nor do they have his wide field of CoO. Sanji will have to improve his CoO to keep up with the Raid suit otherwise he won't have the same effective range on his CoO.
Sanji without RS- broken leg kicking Vergo, face totally fucked up from his brothers, forced to his knees by blocking Judge, so on, so forth
Sanji with RS- punched through a few streets, barely hurts him
He was severely damaged going into the Vergo fight, and his durability will naturally be enhanced as it is for all fighters each arc. He managed to stop Vergo from achieving his goals and killing those marines, it was Sanji's moral victory. He beat yonji no diff and would have probably beat Judge if he didn't cheat, the others were fighting him when his hands were literally tied.
How would you improve with something that makes you better? To improve, you need to overcome your weakness in something. Zoro had weakness with taming Enma and Haki drain, he overcame it. What weakness with RS does Sanji have? And how CoO will help him to avoid CoO detection?
I don't give a fuck about cutlasses. Zoro exchanged Enma with Shusui, the sword with the same rank, how is this a power-up? And his Haki power-up won't become weaker with "cutlasses".
Look up overspeed training... If he is going too fast for him to properly react to, then when he can react he will have improved if he is moving at "light speed" and gains "light speed" reactions to match then he will have improved in base.
If he is invisible, he needs to mask himself from CoO detection or it is worthless, therefore his stealth capabilites will be improved in base as he learns to hide intent-
These are just proposed mechanisms for how this COULD work, Oda could keep Raid suit till EOS, or he could decide to use it as a training tool in these ways I've outlined and it would be believable, especially as Oda is a genius writer and I'm just some internet idiot.
Sanji's hypothetical CoO gains won't go away if you take the raid suit away either. Narratively speaking he either outgrows the raid suit making it's buffs minimal, or he keeps the raid suit till EOS, in which case you can argue reasonably that the strength he would lose would be the same as if Zoro didn't have access to his ultra rare top tier swords which absolutely are a buff compared to random cutlasses.