Market share between Intel and AMD has historically swung wildly on performance differences of only 10-20% which is the kind of performance increase Intel's process can deliver to A9X which at the moment is at or just below Core-M performance. Beef it up to Core-U performance and different thought processes will go through potential buyers minds. The gain in foundry profit would not be worth the potential a.s.p/brand damage done to Core.
You have to appreciate that Intel basically has a very profitable but always potentially fragile business model, it is continually dependent on combined design/process IA x86 superiority enticing buyers to pay a lot more for its chips than the competition. There is nothing preordained or permanent about this and could come crashing down quickly if ARM or even AMD unveil a killer competitive chip e.g. see the damage Athlon64/Opteron temporarily wrought. Why dull this current combined performance advantage by fabbing your direct competitors ? It's just not worth the extra incremental ~$10 die profit.
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