Re: Interesting niches...
As others have noted I seem to recall it was more Apple telling IBM to sod off as POWER chips just weren't really efficient enough for desktop use - hence why the latest and greatest Power PC machines had to be water cooled.
Even if IBM did tell Apple to sod off - now speaking hypothetically - if that was the case I suspect the reason behind it would have been that the development costs for the desktop range of POWER chips simply wouldn't have been worth the income. The development and production costs for processors are insane and, despite Apple flogging about 5-6 million Macs a year, I doubt that was truly enough to build a business case to continue the processor line. Given that we're going a way back and Apple hadn't reached fever pitch I doubt Apple actually sold 5-6 million POWER-powered Macs at the time either.
I disagree with IBM's approach on many things, but business is not as simple as "IBM threw away X amount of business so they're mental" - if that business was not profitable for IBM, and could not be made profitable within a reasonable period (if at all) then binning it was absolutely the correct decision.
Also don't be daft, Apple are nowhere near Intel's largest OEM partner. Looking at the number of PCs shipped they're 5th in the world. Ok so yes many of the other companies also supply AMD processors in their systems, but they are in the minority compared to the number of Intel boxes shipped and even if you say something extreme like "40% of all Lenovo/Dell/HP PCs are shipped with AMD CPUs" (which is just not true) they're all still considerably larger than Apple in terms of Intel sales.
The figures also don't include servers, storage controllers, embedded systems etc that many of the other vendors manufacture. Half a million server units might not add much to the units sold but in terms of value to Intel: servers have more expensive chipsets, (typically) more and (typically) higher value processors and Intel also have a significant share of the 1Gb and 10Gb server-grade NIC market.