Sanji (no DF included) is the most versatile character in OP, you can practically give him any DF and it can work in some way.
The problem with Swim DF is that the stealth abilities it gives is too niche/one way and have very few applications, too dependant on situation.
From the very first arc until now, thay fruit can only be applicable in a few scenarios. All of which can be substituted by similar functioning df.
The other four df plays with those character's strength and traits, and simply befitting their arsenal. Swim fruit ONLY extend one of many Sanji's abilities, it doesnt help:
- His main role, cooking
- Flying capabilities
- Fire/elemental damage
- Kick/fighting style
- Hero acts/saving people
Oda used the "stealth" reason because he wants to degrade Sanji even more with the "peeping" abilities of Swim DF.
If he truly appreciate Sanji, Phoenix DF fits Sanji more, because it can help all of those traits I posted above (also nice opposition with Zoro's azure dragon).