They're not irrelevant, they're only irrelevant to you because you expect everyone to be a fighter. The SH crew are not fighters. They are not even Real Pirates. They are literally just a bunch of friends who met, and decided to sail, and pretend to be Pirates and are seen as Pirates because they end up fighting other Pirates (and the WG). And they are not a part of the WG, so by default, they're considered Pirates, they're not Pirates nor are they even a proper Pirate crew.
What the SHs are, are just people chasing their dreams. One wants to draw a world map, one just wants to find treasure because it's an adventure, one was to be brave, one wants to meet a FISH, one wants to know the world history. These are not Pirates, they don't have ambition to take over territory, or steal, or do Pirate things.
The crew doesn't even function like a Pirate crew nor is it structured like one - there are no commanders, there is no hierarchy, they don't even have plans.
SHs are like tourists, group of friends who went to some remote island, and because some native was kind to them, and the owner of the Island assaulted them, these friends beat up the King, and the entire island's political system changed. These friends left and didn't know what they'd done, they just thought they beat up some mean peasant.
In fact, if you want to define a Pirate crew, they are essentially a business. A business that builds wealth, riches and influence by any means necessary and possible. They are a business that utilise force (military capacity) to get their way. The SHs are NOT Pirates - they just end up in things they shouldn't be involved in like a group of drunkards after a night out, no plans to do what they did, but it happened.
The entire reason the SHs have the Grandfleet and make allies with people who are pretty much people capable of being Captains is because... the SH crew are not written to develop that way. They were written to pursue their dreams while others do the fighting, so it is narratively required to have people that can fight while the SHs fodder can just be themselves and chase their dreams and be comic relief.
Who were the main fighters in every arc since the New World? It has NEVER been Nami, Usopp, Brook, Robin... it's always the TRIO and then the allies they meet during the arc 99% of the time.
The SH crew remains relevant by sticking to why they joined Luffy, to pursue their dreams, and they're still doing that. And they're still pretty much appearing in every chapter.
Just because they're not fighting doesn't mean they are irrelevant. They aren't meant to be the top fighters.
Nami is not going to be fighting some Female Holy Knight, she uses a pole to fight. She doesn't have Conqueror's Haki. Who do they have that can fight?
- Yamato (CoC user)
- Sabo
- Dragon
- Momonosuke
- Scabbards
- The SH Grandfleet - 5600 strong IIRC
- Even the Worst Gen are allies who will probably fight the Holy Knights with Luffy at the end of the series - Law, Kid, Bonney, etc.
- Hajrudin, Loki
There are countless people with King's Haki that are associated with Luffy, so why do we need Nami and Usopp to use CoC? We don't, it's not required for the story to develop.
Nami, Usopp... literally all the SHs can achieve their dreams without needing to fight. What this story is doing is simply showing us their journey to achieve their dreams.
When was the last time we saw a SH member with their own ship or fleet? Never, their role was never to be relevant in fighting. Otherwise, they wouldn't have so many allies and the crew would actually be organised... they're none of those things. If they manned their own fleets and such, then it'd make sense to be the best fighters and have Armament at least, but they're not supposed to do that, that's mostly for the trio.
I think this is an issue of having the wrong expectations.
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