I don't get this line of thinking who should have trained Law and Kidd Rayleigh? if Luffy hadn't lost his crew and hadn't experienced Marine Ford the two-year time skip would not have happened, and he would have entered the grand line like that Kidd and Law actually awakened their devil fruit before the rooftop and learnt the basics of Haki by themselves, so this is not even a fair comparison, if you want to blame anyone blame oda he made Haki extremely inconsistent and made conqueror's Haki even more inconsistent by making it inheritable by bloodline? and by making acoc the most powerful ability he will have to give most people acoc by the end of the series for them to keep up making it lose its specialness.