Big Mom started the Yonko system when she started conquering territories, Shiki Roger and Whitebeard didn't care about territories and were just called great pirates
Whitebeard was called a yonko, but only after Roger's death
Which leads me to believe that the yonko system is fairly modern. There was no yonko in the roger and rocks eras because there was an imbalance of power. Rocks and then Roger were both above the rest of the great pirates and unlike WB, posed a serious threat to the WG due to their actions.
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The power of retcons, bepo in a few arcs will be > king.
I feel like Bepo will probably reach that level eventually.
The SHs are basically an anomaly in how strong they've gotten in such a short period of time. For most other characters, they don't reach their primes until middle age.
If Bepo chills with Law in the new world for like 20 more years, I could easily see him peaking at YC1 level. Eventually he would come across a powerful DF, awaken haki, and master su long.