it would have costed Luffy zero dollars to go snakeman and smack kanjuro down with a jet culverin. tbh I feel like maybe Luffy was fine with Momonosuke being captured. I mean he didn't even seem worried about him or anything.
I'll just be upset if Momonosuke's capture ends up being a major point of tension in wano, given how careless luffy was when it happened. I mean Luffy can fly, he can throw long range punches, and he can even see the future. he could have saved Momo if he wanted to.
Maybe Oda just activated plot induced stupidity. But I'd like to think that Luffy simply had confidence that Momo could handle himself or that even if Momo was captured, they could easily get him back by having Sanji go stealth black or Law use room or something.