Firstly:
1st - The g4 lasts much longer than just 1 minute...
2nd - Luffy does not seem to have deactivated his G4 on his own after all he seems perplexed by the fact that he is "losing his powers".
3rd - The g4 lasted a long time against Katakuri, against cracker, against Doffy, but it didn't last even 1 minute against Big Mom, and this coming from a Luffy that was 100% of his energies and Haki.
Let's stop being idiots, there is no problem in Big Mom being able to deny Luffy's G4 using her Haki, after all she has a monstrous Haki capable of that...