In terms of operating systems I doubt anyone new could enter the market (as in build a new operating system), its just too expensive. Linux would take off, Solaris would resurrect in the server market and Apple would just explode.
AMD would be screwed. Intel would be just fine with Apple.
Don't think I can agree with you on either point.
First of all, building an operating system is NOT all that complicated. There have been many many of them, even not including BSD and Linux variants. Why haven't they made a deep penetration? Because MS has a freaking monopoly and has used very questionable tactics to squash them. Where would OS/2 or BeOS be now if MS were gone? What about Atheos/Syllable, which is not Linux or BSD. DrDOS/GEM? GNU/HURD?
Second, if there was no MS, there are plenty of OS to fill the void which can run on Intel and AMD, so AMD would not be "screwed" unless they couldn't create products that could compete with Intel.
Apple isn't the only other game in town, and would not become the next MS if MS were gone, despite the fact that (IMHO) that's Steve Jobs' fantasy.