Nmixx debuted with one of the most unacceptable songs from a big 4 group, according to the public. Consequently, people didn't have high expectations for them. Fans who stan them are either genuinely in love with their songs or are simply company stans. They lack success stans or a general public following is actually a blessings in disguise as public reactions or 'quality' of their songs won't lead to a significant downfall.
On the other hand, with Itzy, they have numerous success stans, who now probably stanning more successful groups, seeking a sense of superiority that Itzy may no longer provide. They have the general public in their favor, and people love their music, image, and concept. Consequently, if they change their concept or if the quality of their songs declines, they will face substantial criticism. They have a reputation to maintain, unlike Nmixx, whose songs have their own fanbase by now.
In short, JYP's strategy for Nmixx is not bad; it's simply a case of not losing what they never had.