Warlords System was added around 13 Years ago
It served to reduce the Number of Pirates, basically as a Counter to what Roger caused
So their Job was to destroy the Dreams of New Pirates (Such as what Mihawk did to Krieg)
And to prevent any New Emperor or Powerful Alliances from emerging
Basically keep World Balance
However, during that time, WB, Big Mom & Kaidou were already Emperor-Like Figures. Shanks wasn't
This means that Shanks is only Pirate who managed to reach Hype Level of Rocks Crew despite Warlord System & efforts of Marine HQ
They must have definitely sent him a Warlord or Admiral to stop his Rise (Similar to how Luffy encountered most of them before Saboady)
And yet they failed, indicating that Shanks defeated an Ex-Admiral or Ex-Warlord