Did luffy stay in the g4 form to have a long fight with it? No. He used it quick to oneshot the number then returned back to base avoiding any of thr drawbacks right? Yes. So terrible example.
He used it against the ship then reverted back into base before drawback could even happen right? A other terrible example.
He didn't use it on ulti because of yanatos intervention however he did state that HE WOULD NEED it against ulti which is why it's a fact that ulti was slept on
I using your logic, if him Uses G4 against a Ship, him will not destroyed that Ship with other forms, right?.
Him used against the number, so him doesn't has other power who him wouldn't be able to deal with him, right?
I am using this same logic as you said that, Luffy needed only G5 to deal with Lucci, and This isn't the truest.
Now I ask you, how Luffy is maintaining more G5 now against Saturn and the Goroseis, but against Lucci him get taxed?