It's kind of tricky, really. After crossing over to the new world, our expectations for the capabilities of new enemies is so high, that for Nami to be able to solo one would just seem insulting, and if it's a bunch of no-name thugs, well, anybody from the crew can take them out instantly, but nobody really thinks much of that (Nami did knock that dragon out of the sky with a thunder cloud, for example, but who cares?) Even the new fishmen pirates were all well-below the skill level of any of the strawhats, but only became a threat due to performance-enhancing drugs.

Back in Water 7, someone told Usopp that, even if he can't fight, he can find other ways to contribute that nobody else can. I'm okay with that.

Edit: Also, sure, everyone has trained hard the past two years. But unless you can say that Nami and Usopp are stronger now than Luffy was two years ago, then it's still not enough for them to reliably solo new world enemies.