While Teach has been obtaining DFs, Luffy has been obtaining advanced haki. It doesn't make sense for Teach to have advanced haki and OP DFs, because then he just has every power and is unbeatable.
Teach doesn't need advanced haki because his DFs already give him the abilities of advanced haki, or outright counter advanced haki. Lets see:
The yami yami no mi's ability to negate DFs is a stronger version of CoA's ability to merely counter DFs. Furthermore, the yami yami no mi's ability to create black holes counters CoO's ability to see the future by pulling the opponent towards Teach, taking away their ability to dodge.
The gura gura no mi's ability to create shockwaves is a stronger version of ACoA and ACoC's ability to create barriers and do internal damage. This DF can destroy the world, no ACoC user we have seen so far can do that with ACoC alone(or at all).
There're a few more haki abilities that Teach's fruit can't replicate. I believe some or all of these abilities will be granted to Teach in a third DF he will eventually obtain.
Anyways, in order to surpass Teach's DF-obtained superior versions of the standard haki abilities, Luffy will need to fully master haki, and in doing so surpass the power of DFs.
Teach doesn't need advanced haki because his DFs already give him the abilities of advanced haki, or outright counter advanced haki. Lets see:
The yami yami no mi's ability to negate DFs is a stronger version of CoA's ability to merely counter DFs. Furthermore, the yami yami no mi's ability to create black holes counters CoO's ability to see the future by pulling the opponent towards Teach, taking away their ability to dodge.
The gura gura no mi's ability to create shockwaves is a stronger version of ACoA and ACoC's ability to create barriers and do internal damage. This DF can destroy the world, no ACoC user we have seen so far can do that with ACoC alone(or at all).
There're a few more haki abilities that Teach's fruit can't replicate. I believe some or all of these abilities will be granted to Teach in a third DF he will eventually obtain.
- extrasensory perception(CoO)
- domination of wills(CoC)
- invisible armor(CoA)
Anyways, in order to surpass Teach's DF-obtained superior versions of the standard haki abilities, Luffy will need to fully master haki, and in doing so surpass the power of DFs.
