For those reasons, I don't like the idea of saying that Shiki was like a Yonko.
Oda said Shiki is one of the great pirates, he just didn't add his name in Chapter 434 as to not confuse readers.
And an Emperor is simply one with great influence, why wouldn't there have been influential individuals pre-Roger?
It's like saying no leaders existed in the past 1,000 years until a certain point; there is always hierarchy.
And Pirates aren't noble, they're supposed to be selfish. Wanting is just part of the nature of being a Pirate, wealth, fame, and all these things are the desires of Pirates. There's no reason why Pirates wouldn't possess territories. Shanks is an Emperor, therefore, he's got control over a huge portion of the New World like every other Emperor, reason why? We don't know, and one doesn't need to be there being that Pirates are naturally greedy.
Even looking at the character of Ace & Roger; as much as Ace hated his father, he wanted the exact same things and did the exact same things, they were literally carbon copies of each other.
- Wanted Wealth.
- Fame.
- Defeat everyone in his way.
It's no surprise that Roger was hated being these were the things he did, and we're told both he and Ace were brought up in similar conditions. Majority of Pirates were probably of the same make up besides their irrational behaviour towards running away from a battle they're likely to lose.
Shiki scoffs at the notion that the Pirate Age has just begun in the sense that, they will rule the Grand Line, they already did in Roger's era. The only change is, there are more Pirates heading for the Grand Line as a result of Roger's treasure, not a new era.
More so, Jinbei is lamenting at the consequences of Whitebeard dying, not just for his beloved Fishman Island, but because the Grand Line which is not ruled by the WG would become all the more chaotic.
Joker himself understands that the sea was never ruled by the WG, which is why he asks who would rule this seas? The Pirates, RA, or the WG? If they were ever in control, then the Pirates should never have possessed territory in the first place. There's a reason why the New World is a Pirate Haven.