Oda still cares about his overall power level structure in One piece , they don’t just need an asspull but a miracle to be even comparable to both Zoro/Luffy , Coby was no diffed both the times he fought with Luffy( Water 7 / Marineford ) , and you expect me to believe that on top of this huge gap just by training with Garp is sufficient for Coby when Luffy himself trained with Rayleigh prior to their previous interaction.
As of now nothing indicates that they will be comparable at all , in future if any new information regarding their powerlevel is shown then one could backtrack but with showings until now Eos Luffy washes Coby and Helmeppo at the same time.
the problem with coby n hellmeppo is they lack of screen time so we dont know how much character developent they got n power up they have ....
maybe they will show up in final war as luffy support if oda remember them ....
unfortunately helmeppo and coby will have to grow unrealistically for them to be competent enough to match either zoro or luffy
but I dont see coby and helmeppo replacing the current admirals anytime soon.
for reference helmeppo is a chief petty officer
and coby is a captain
not even a rear admiral at this point in this story in parallel to luffy taking on kaido .
unless there is a timeskip coming soon
a couple months to the end of the story
that is unrealistic.
it is interesting just the two of them have marine" rivals" tho
we know who the marines take seriously in the near future.