They will never be useless.
They're more energy efficient moves for Luffy now, and the more his mastery over his DF Awakening grows, the better he'll get at managing his energy.
Luffy will always have a use for a burst of G2 or use for G3.
Goku hasn't stopped using Kaioken or lower forms of Super Saiyan just because he's got Ultra Instinct, has he? It's the same for Luffy. His arsenal isn't shrinking, it's multiplying exponentionally. He'll have different levels of intensity and precision to dial for all of his attacks.
All of this happening is very much a good thing for Luffy. The more casually he uses G4 the better, it's just a sign of how much stronger and better he's becoming.