I am an avid Zoro fan, but I have to agree that Zoro is definitely the most replaceable member of the SH pirates. Let's see what purpose each crew member fulfills:
- Nami: Navigator, probably the most important component of a crew.
- Ussop: Sniper, technology manager and spare shipright. He was the one who kept repairing the Merry Go when it was getting damaged repeatedly. I will put him as the sixth most important component of the crew.
- Sanji: Cook, probably the second most important component of a crew.
- Chopper: Doctor, normally he would be the third most important person in the crew, but I will put him at fourth.
- Robin: Archeologist, not useful in a pirate crew, but seeing as how Luffy's goal overlaps with Robin's, and seeing how Robin is an one-of-a-kind scholar, she is the third most important component of the crew.
- Franky: Shipright, obviously he is also very important to the crew. I will put him at fifth (only because Ussop the spare shipright is present in the crew).
- Brook: Musician, well...he is not all that useful. But he serves a purpose of providing entertainment for the crew. But he appears to be excellent in espionage. His combat abilities are also very good. Let's put him at eighth.
- Jinbei: Helmsman, I don't think Jinbei belongs to SHC. But Oda has created a purpose for him and touted how lack of good Helmsman was a significant issue. But since the SH pirates managed without him before, I put him at seventh.
Now, let's look at Zoro. He is the swordsman of the crew. A crew can function without a swordsman. His contribution to the crew appears to be:
- Combat
- Support to the captain
Now, Nami and Sanji has also acted as support to the Captain too. Combat doesn't make him an indispensable part of the crew either, specially in a crew where Luffy, Jinbei and Sanji exists.
So, ultimately, Zoro is the most replaceable SH pirate. He doesn't share any link to the final goal (Laugh Tale/Changing the world) and he doesn't contribute any skills to the crew.
Now, what does that mean? Well....nothing. Luffy and Zoro are the founding members of SH pirates. The crew was formed when Zoro agreed to join Luffy. The other characters came with the intent to fulfill some specific purposes (except Robin, who is a stowaway).
So, is Zoro needed in the SH pirate crew? No. But, there is no SH pirates without Zoro. Others came because Luffy and Zoro founded the crew to begin with.
This should be the takeaway from this.