I think Warlord System was to Stop emergence of New Emperors
That's why They send a Buster Call or Admiral immediately in case a Warlord loses his Title
So these Warlords are helping them stop Golden Age of Piracy & Crush New Pirates, but at same time, it's to not let them create Yonko-Like Empires, that's why it's not allowed for a Warlord to create Alliances or create Large Organizations like Baroque Works (Which Operated in entire World)
First Warlords were Mihawk, Crocodile & Moria, and i'm gonna assume Shanks was Invited too but He refused
Moria was considered by People to be a Rival of Kaidou, Crocodile was probably Most Famous Supernova of his Time, and Mihawk was already a Legendary Swordsman, alongside Shanks
Hancock & Jinbe were also Recruited to keep Kuja & Fishmen from becoming a Bigger Pirate Empire (Imagine if Jinbe was like Hody & was able to recruit the entire Island, They could have invaded hundreds of Islands), and also look at Blackbeard & Buggy who indeed became Emperors, and Weevil who is WB's Son, also Ace who was Roger's Son & was too strong for a Rookie ... etc
So you could say that one of reasons Warlord System was created is to Stop all of them from Rampaging, cuz They represent "Yonko Candidates", even their Bounties demonstrate that, Hancock for example was given 1.6B and Crocodile 1.9B, which are Greater than 1.5B that was deemed fitting for Yonko Candidate Luffy
However, their Bounties also indicates that Mihawk was considered the Biggest Problem among Warlords, hence his very High Bounty, even compared to New Emperors, if Mihawk refused Warlord Title & instead started a Crew like Shanks did & operated in New World for Years, He would have easily reached WB's Level of Influence & Surpassed Kaidou's Bounty
