Just remember the bigger you get the harder you fall, it has happened once.
um... when has it happened to apple? they've never been dominant.
regardless, i don't see it happening anytime soon. they have at least 4 core markets, and they're solid in each:
1. desktops. not dominant in the market, but dominant in their niche. high quality. not much room for market movement, up or down. no risk of backlash.
2. notebooks. not dominant, but very good, and on the upswing, there's not a lot of room for movement up or down here -- so again, not much to lose.
3. mp3 players. i guess we call them "iPods" now. not only totally dominant, but has tons of lock-in: AAC content protection, iTunes, computer halo effect, current cool factor, brand recognition... there's so many technological and cultural issues working in AAPL's favor here it's almost ridiculous. amazingly, they even have room for growth here into the video market. one's tempted to say that there's nowhere to go but down for the iPod segment, but it's at the same time difficult to see what the weakness is.
4. iphone. expectations were and are high, but here, there's not really anywhere to go but up, now that we're past the initial reception. and, in addition to the ipod-type lock-in, there's a 2-year contract lock-in here.
two really amazing things about that list:
1. independence. each of the core businesses is probably profitable on its own (although the margins in the desktop market suck), and there's not much room for loss in any of them -- market share might fall, but AAPL doesn't have its neck far out on anything. --and considering their COH, they could probably weather storms even if this wasn't the case.
2. synergy. each of the four areas tends to reinforce the others. not just the computer <-> portable interaction, but itunes, isync, etc. more to the point, it's very easy to see how each area falls within a core competency. (unlike, say, the XBox...). as far as moving forward, most advances on one front (even R&D and design) tend to have effects across the business, so there's fewer risky internal decisions to make -- not that Jobs' record on that score has been bad...
to top it all off, AAPL isn't facing a major threat from any of its direct competitors (and by some accounts, some of them are floundering independently -- but given the recent quarterly reports, we don't need to go there) and appears to have a bunch of tricks left in the bag (Leopard is obvious, but expanding iPhone coverage and models is also an easy one, to say nothing of all the patent apps, potential 'ultraportable', etc.). they have no obvious weaknesses. their track record so far has also been astonishing.
in short, your "bigger they are, harder they fall" objection is an interesting cliche and worth keeping in mind, i suppose -- but unless you have more specific objections, it's a pretty empty notion.
/fanboy off