The way I see it Mantle wont replace DirectX, it can't (technically it could, but other factors mean it can't).
Someone mentioned that G-Sync costs money but Mantle is free. Mantle is only free if you have an AMD card, otherwise you have to buy an AMD card. If you have an AMD card G-Sync isn't really an option, unless you buy an Nvidia card. Even then you'd need to buy a G-sync Monitor regardless of what card you own.
G-Sync does have the advantage, if it doesn't need to be specifically supported by the game. Mantle has to be coded into the game along side DirectX. It doesn't need DirectX, but the developers (or rather the companies employing them) do. Your game sales wont be great if you exclude every Nvidia card, every Intel 'card' and the large number of AMD cards that don't use GCN. That's eliminating a large number of your customer base before you even take into account who actually want to buy the game. Mantle could only replace DirectX in a world where such a large majority of customers own a recent high(-ish) spec AMD card that the games company doesn't care about the remaining people.
Personally, and I say this owning 3 GPUs that support Mantle, I hope that Mantle quickly dies a death and some real work is done by all GPU manufacturers to OpenGL as powerful and efficient as possible.
Regarding the CPU aspect of gaming and how games are using more threads. The thing is if a game uses multiple threads but still uses a single thread for the 'main' process and uses other threads for background tasks or such then you still need good single threaded performance on the main thread. If Mantle can balance the threads so that usage across cores is more even that that should help.
Also, anyone saying AMD CPUs, specifically Bulldozer and Piledriver CPUs, are good for gaming obviously haven't played GW2. That game hates those CPUs. My Phenom II X6 1055T @ 3.8GHz outperformed my FX-8350 @ 4.5GHz by probably close to 50%. My 4770K @ 4.3GHz is well on it's way to giving me twice the FPS of the 8350.
So guess it depends what games you play! I believe the original Crysis only uses 2 cores/threads. And yes, I sometimes like to play games that were released more than 4 days ago, so shoot me!
As for the 290X, the cooler does seem off-putting. However, after the 5870 I decided not to buy AMD cards close to launch. Not because their release prices are unreasonable compared to the competition (except maybe the 7990), but because AMD seem much more willing to drop their prices. So I'm going to (hopefully) run 2 or 3 7950s for a little while and see what happens. Maybe the 290X will have some better 3rd part coolers then and even more reasonable prices. Plus we might see what effect Mantle has and hear more about what sort of support it will get.