Sure, Oda can use First Arc to give us a "Taste"
But Egghead showed us way more than just a Taste
We've seen Mother Flame Used, so it wasn't just a Taste, We've actually seen how Powerful is their Secret Weapon
They sent CP-0 and not just any Non-Named Masked Agents, They actually sent Three Major Characters & definitely saw them Fight
They didn't just send some Marines, not even a Normal Buster Call, They sent 20 Battleships alongside 80 Smaller Ones led by around Ten Vice Admirals, not just 5, and even brought an Admiral, and They been Fighting for so long now
They didn't just Tease New Pacifista Models, They actually showed the entire 50 of them, and They were shown Fighting both Battleships & Destroying an Island & we kept switching between who commands them & then They got all Defeated by Venus
They didn't just Tease Seraphim by showing One or Two, We are seeing Four of them & They already appeared many Chapters & their DFs were Revealed & They are still Part of the Conflict & Fought multiple Characters
So yeah, Mother Flame, CP-0, Marines, Pacifista, Seraphim ... etc will still appear Post-Egghead & have Important Role, but this is far from saying it's just a "Taste", Oda could have shown much less than this if They are supposed to only be Teasers, no need to make the Conflict this Big when the SHs aren't even in Mood to Fight
And finally, We have Gorosei, at first We could agree that Oda was just showing us a Taste cuz We only got Saturn, but All Five? That's more than just a Taste!
When Oda wanted to show us a Taste of Crocodile's Commanders, did He Introduce all of them in Whisky Peak or Little Garden?
When Oda wanted to show us a Taste of Kaidou's Calamities, did He Introduce all of them in Zou or only Teased Jack?
When Oda wanted to show us a Taste of Enel's Priests, did He make SHs face all Four Trials at once or only Satori's?
When Oda wanted to show us a Taste of Sweet Commanders, did He make all Three attack Luffy in Seducing Woods?
All Five Gorosei are in Egghead, Mother Flame is in Egghead, Majority of Seraphim are in Egghead, All Pacifista are in Egghead, Only CP-0 Masked Agents who are Relevant are in Egghead, and their Biggest Victims are in Egghead, Kuma is there, Bonney is there, Vegapunk is there, Ancient Robot is there & who knows who else gonna come too
Oda doesn't need to keep these Enemies, this is the Biggest Role They ever gonna get, They will appear again, but They will be Less Important than They are now