A lot of people think Oda somehow retconned G5 because Luffy recharged himself against Kaido but not in Egghead. These people seem to miss something very important.
Read this dialogue
Momo: His voice is still fading quickly
Kaido: Hey, you're really going to kill yourself
Luffy: after all this, do you really think something like death can scare me?
Yes, Luffy could have revived himself using G5 and gone on to fight Saturn. But doing so would have put his life at risk. While something such as this was necessary against Kaido, it wasn't necessary in the past few chapters. Luffy still has his haki, he can use FS and he knew that nothing was going to happen to Bonney. Were he to see a future where Bonney would have gotten seriously hurt, I'm sure he would have used a second round of G5.
But apart from that he really had no reason to not recover his stamina before fighting some more. Especially since Saturn is at full health.
Read this dialogue
Momo: His voice is still fading quickly
Kaido: Hey, you're really going to kill yourself
Luffy: after all this, do you really think something like death can scare me?
Yes, Luffy could have revived himself using G5 and gone on to fight Saturn. But doing so would have put his life at risk. While something such as this was necessary against Kaido, it wasn't necessary in the past few chapters. Luffy still has his haki, he can use FS and he knew that nothing was going to happen to Bonney. Were he to see a future where Bonney would have gotten seriously hurt, I'm sure he would have used a second round of G5.
But apart from that he really had no reason to not recover his stamina before fighting some more. Especially since Saturn is at full health.