Shanks is the goalpost for Luffy. Luffy surpassing Shanks would fulfill a promise Luffy made in chapter 1.
This is why Luffy was only given a 3 billion bounty. Oda didn’t want him to surpass Shanks’s 4 billion.
However, I speculate that from Egghead, Shanks will be given a 5 billion berry bounty for destroying Kid(low diff), who was 3 billion. Luffy will surpass 4 billion for beating Kizaru(extreme diff).
This will make it to where Luffy’s ultimate goalpost is in the 5 billions. So when Luffy surpasses Shanks, he will surpass Roger as well.
This is why Luffy was only given a 3 billion bounty. Oda didn’t want him to surpass Shanks’s 4 billion.
However, I speculate that from Egghead, Shanks will be given a 5 billion berry bounty for destroying Kid(low diff), who was 3 billion. Luffy will surpass 4 billion for beating Kizaru(extreme diff).
This will make it to where Luffy’s ultimate goalpost is in the 5 billions. So when Luffy surpasses Shanks, he will surpass Roger as well.