I agree with Networkman that Apple is quite the niche player. I won't go so far as to say they can't sustain themselves being that niche player though.
I believe th ekey's to Apple's long term success are:
(1) Maintaining the educational market
(2) Gaining significant market share in consumer desktops
(3) Ignor medium, large, and enterprise companies outside professional design industries (media, design, advertising, marketing, other creative)
(4) Ignor servers other than media server software (quicktime, etc)
If they can achieve these goals, Apple will be around becuase it will be the preffered family computer (simple, easy to use, astheticly pleasing, what the kids use at school, etc) and they will not have spent precious dollars (just becuase you have lots of cash doens't mean you should waste one cent) on the corporate market with significant barriers to entry.
I think the world could co-exist with Macs at home an PCs at the office (especially if those PCs are running Linux, a far more stable, secure, and faster OS than Windows).