I hate haki so much...
It was never even that necessary. The only fruits that didn't have obvious weaknesses were Smoker and Kizaru, but with a little creativity, beating them could've been possible without haki. I mean, if haki can just cancel devil fruits, then why did Blackbeard, a powerful haki user, go through all that trouble for the yami yami no mi? Why was Ace so shocked that Blackbeard could hurt him? It was obvious haki wasn't planned until around Sabaody, and Mantra was clearly meant to be some skypiean race exclusive ability rather than observation haki.