The coby stuff was there from since the start of the manga .
Force how ?
We saw coby grow the same as luffy but not as crazy fast but still fast .
He was always going to be the new face of the marines and has been build up that way for decades .
Side note if you can keep haki in bullets , arrows etc etc for who know how long away from the user guess this is like part of that .
But this with AK tech .
I say it's forced cause it doesn't make sense.
Coby started from a level weaker than kid Luffy, 10 year old Luffy! I can buy him becoming strong maybe like 10 years into the future in epilouge, but not at current timeline.
His persona also doesn't fit a conqueror's haki user at all, which would be a requirement if he has to become a DF-less top tier.
Coby is also a massive Luffy fanboy
Imagine Garp being Alvida's bitch slave and giga fanboying Roger. Pathetic.
Smoker on the other hand, was THE marine antagonist of Luffy for the entire pre timeskip. He was stronger than Luffy for the entirety of the pre TS, has displayed very high will power (first marine shown the actively go against the marine orders), and has the demeanor fitting of the series' marine antagonist.
His progression to being the leading figure of marine rebellion would've been very organic and natural narrative progression of the character.