At 20nm they will. Not everybody will move to 20nm due to the cost and complexity so there will be more available capacity.
Can Intel be successful in the foundry business? Of course they can but it will not happen anytime soon. It took Samsung 10+ years to get to the number 4 spot. Today Intel Foundry uses the ASIC business model like Samsung did in the early days. Customers throw RTL designs over the Intel wall for physical implementation. This helps Intel learn the SoC foundry business and it protects Intel process secrets. Moving forward Intel will have to develop a fabless semiconductor ecosystem (exposing process secrets) and forge EDA and IP partnerships with the likes of ARM.
Intel will also have to avoid the competing with customers conundrum. The Intel UltraBooks are a blatant copy of Macbooks. The Intel Atom will someday compete with ARM and don’t be surprised if Intel comes out with an SoC of their own. Sounds a bit like Samsung right? Deja vu all over again. TSMC on the other hand is a pure play foundry and does not compete with customers.
My bet is: moving forward Apple will use Samsung for 28nm (iPhone 5s) and TSMC for 20nm (iPhone 6). Intel certainly has a shot at 14nm and 10nm but never ever count out TSMC.