i don't expect Akainu to be a direct antagonist for the Strawhats until very near the end of the series.
In the meantime, I expected Smoker and the G5 unit to represent the Marines ongoing attempts to harry the Strawhats.
Personally, I think this would be more effective from a storytelling perspective if Smoker's unit had more actual named characters in his unit. Just him and Tashigi doesn't make them a credible threat. But maybe with Hina, Fullbody, Jango, Coby and Helmeppo, with 2 years of increased abilities, they'd at least feel like a faction that, I dunno, mattered.
Right now, they don't.
Through a plot device obviously.
There are other ways to get powerful besides just training after all.