Sure luffy used them once and you can argue it was comic relief but you are disregarding a key fact here - his personality. He comes up with these often...
He used G3 despite turning having a downside of him turning into Chibi
He invented G4 after seeing giant animals
Just look at tankman and tell me this isn't ridiculous for you
You people are too used to seeing some cool PUs that all it matters for you all is how badass it looks. That's it.
G5 fits with Luffy's personality just like how KoH fits with zoro.
And, another point you all ignoring is one piece is DF world - ridiculous abilities will exists.
I talked about creativity - G5 does leave room for creativity and we have seen it how he fought kaido *cough* jump rope*
And, yes luffy defeated kaido with giant punch. So? What else were you expecting from a brawler? Sword slash ?