I wasn't aware that there exist some people who believe that Admirals are over Yonkos. I thought the story made it pretty clear when they needed 2 of the 3 great powers to stop one Yonko with the title of the strongest man alive and that monster had to be nerfed by mulitple angles such as age, illness, backstabbing, etc.
If that wasn't enough Garp stated at Mariejois, in the events that 2 Yonkos would team up, it would exceed their imagination(marines). Practically saying that no matter fear you have right now of this scenario happening in the near future, its pointless because it will be worse than what you can conceive. Oda is hyping up Kaido and Big Mom as the next level threat after Whitebeard in Marineford, plus Kaido has another title of the strongest creature, which means he should be stronger than Marineford Whitebeard.
Yonkos have received the most hype in the entire series, they stand at the top of the food chain and by default Top tier.
First mates are given special portrayal compared to other Yonko commanders, so i think they shouldn't be grouped in the same class, thus the need for tiers.
I don't mind no tier either, but it should be at least.. Yonko > Admiral > First mate > Yonko commander
Then again, tier are necessary for scaling someone such as Mihawk and Dragon, since there are no concensus on their power level.