They are not spectators as they are actively agents of change. The only arc where thay arr spectators or witness is maybe Zoo.
Unless you can give me an arc where they have been more spectators than actors.
If you remove SHs from Egghead Arc, 99% of Events wouldn't change,
CP-0 were still Coming,
Saturn & Buster Call were still Coming,
Lucci & Kaku would still be taken down by Stussy,
Seraphim would still manage to kill few Vega Satellites,
Kizaru would still Kill Vegapunk,
He would still feel Sad & take a break,
Kuma would still Save Bonney,
Bonney would still Control Pacifista, and Buster Call is Triggered,
Vegapunk Broadcast would still happen,
Devon would still steal Saturn's Appearance,
Gorosei would still be Summoned to Stop Broadcast & They'd Fight Ancient Robot,
... etc
Story was Modified to give SHs a Role, such as making Luffy/Zoro Free Lucci just so that later Oda can give Zoro a Fight,
And SHs were Busy protecting Satellites, even though They are gonna Die anyway, and will Escape later & be Re-Created,
... etc
Only SH that was needed in Egghead Arc was Luffy for his Sun God Identity, which is Important for Kuma/Bonney's Character Development, Emet deciding to Unleash Joy Boy's Haki, and Dorry/Brogy taking SHs & few other Characters to Elbaph,
And guess what? Elbaph is like already 40 Chapters & SHs are still just Spectators,
This means for Last 100 Chapters, SHs were Mainly Spectators,
And for 100 Chapters after Sabaody Arc, They were Absent, it was only Luffy Piece,
In Zou Arc, They were also just Spectators,
In Whole Cake Island Arc, Half of them were Absent, while other Half was Absent in Dressrosa Arc,
And aside from forced few Fights, most of them weren't needed at all in both Arcs,
They also were mostly Spectators in Wano Arc (Act 1 & 2), it was until Onigashima Arc that They started participating more
... etc
The idea isn't that Oda doesn't come up with a Role for SHs in every Arc, He obviously does,
But these Arc Stories are not written for them, SHs are the "Uninvited Guests", and then Oda create situations for them to look helpful or needed,
Egghead Arc didn't convince you, check Onigashima Arc too,
Did SHs Defeat Army? No, The Goal was to Defeat Kaidou & it was done by Luffy,
What were other Major Problems?
Orochi? Killed by Kyoshiro
Big Mom? Defeated by Kid & Law
CP-0? Delayed by Izou & Drake
Flying Onigashima? Stopped by Momonosuke
Suicidal Kanjuro? Stopped by Yamato
Ice Oni Virus? If not for Marco, Chopper & Minks wouldn't have time to develop a Cure,
Smile Army? It was Countered by O-Tama
Ulti & Page One? Defeated by Big Mom
Hawkins & Apoo? Fought Killer & Drake
Jack? Defeated by Inurashi
Perospero? Defeated by Nekomamushi
WG Invasion? Stopped by Zunisha
Fire? Raizo provided Water to stop it
... etc
Oda didn't need SHs at all, They simply had to be present there, so He created some Commanders to keep them occupied,
(During Arlong Arc, SHs were only Four, so Arlong had only 3 Commanders, but against Hody? He created more Commanders)
They were Useful, but to consider them Main Cast of every Arc? Nah, not even Close,