I agree with you, but to play devil’s advocate, the blue one symbolizes Luke accepting Rey’s request for help (and his “role in all this”) that he rejected by tossing that same lightsaber aside at the beginning of the film, so changing it to the green one would lose that bit of symbolism.
But again, I’m with you and would prefer it to be the green one, however that would require hilt replacement too, and I don’t know how feasible that is.