Bounties are a mess, so trying to use them for that purpose doesn’t even work.
Nothing states that the original 4 became Yonko with the bounties they have now or right before they died. Shanks the youngest has been a Yonko for 6 years. The others have been pirates for even longer and racking up bounties since youth.
Blackbeard has been a Yonko for only 2 years, which is 3 times less than Shanks. Yet his bounty is almost the same. Luffy and Buggy have been Yonko for less than a month. Luffy’s bounty is suppressed and should be around Kaido’s.
The difference between Blackbeard’s and Shanks is less than between the latter and Big Mom’s who was 68.
Besides that, it’s all just an improvised mess by Oda. 1.5 billion was supposed to be for the “5th Emperor”
I agree that bounties cannot be used for outright powerscaling.
There is still some logic to it, though.
Shanks has the lowest bounty among the OG group because he is the youngest and wasn't really all that of an active threat. Guy is a party monster, not a monster monster like Blackbeard.
Whitebeard is the highest despite not being a psycho, simply because he was basically an uncrowned Pirate King and an old fossil so he had time to accumulate the bounty.
Big Mom and Kaido are in between. Not as old as Whitebeard and likely not as strong as Shanks, but both were psychos and quite active in the pirating world.
Blackbeard has bounty almost equal to Shanks despite being Yonko for much shorter period of time because he a) is actively hostile and b) has killed Whitebeard in full view of the world (yeah, he needed help to do that, but still...).
Buggy is useless AF, but the Marines don't know that, and his Cross Guild is
actively hunting Marines, making him much more of an active threat than the likes of Shanks or Luffy.