See Berkshire Hathaway. Share price ultimately means nothing (because most companies typically split above a certain level, historically at least, to keep prices in a sweet spot). Apple isn't the first company to chart those sort of gains, even after being on the verge of bankruptcy. Agree they aren't going anywhere soon, but they likely won't continue to be the dominant market player. Of course, as you point out, they don't really dominate any of their markets, aside from tablets and maybe mp3 sales (if you exclude Pandora, Spotify, etc..). It's a great business, no doubt. But the last time they lost Steve Jobs they went into a nosedive.
They've done a remarkable job of maintaining margins, but tech hardware ultimately becomes commoditized. There's still a huge business there in software and digital media but the hardware margins will start dropping. Already seeing that on the Android side of tablets. Smartphones only remain at the prices they are because of the carrier subsidy or they'd start dropping like a rock, but that's a model with no indications of changing.