This is not nearly as big of news as it sounds.
Firstly, Steve is not leaving Apple, he's 'just' going to be the chairman of the board, and his right-hand-man, who has effectively been running the company for a few years now, is taking over. This is just the continuation of a long-running trend of Steve stepping back for the last two or three years.
Secondly, Apple are not losing anything. You can make up a statistic to make Apple first or last by just choosing what to count. iPhone -v- Android is a great example of a false comparison like this. Android is an OS, the iPhone is a single product, surely you need to include ALL iOS devices in the stat? That means adding in iPads and iPod Touches too, both fantastic selling products. Another way to look at would be to make the comparison fair by picking the single most popular Android phone, say the Samsung Galaxythe, Droid, and comparing those to sales of the iPhone. If you do that, then you find that the iPhone is the most popular smart phone by a mile. Bottom line, companies are about profit, not units, and Apple are king of the hill when it comes to profit share in the consumer smartphone, tablet, and computer markets. Nokia sells more phones, but it makes much much less profit.
Apple is also creating an entire ecosystem which works together harmoniously, and that's really going to go up a notch with iCloud later this year. All iPhone and iPod and iPad customers will be strongly incentivised to buy more Apple products because of how well they all work together. No one can compete with Apple on that score, because no other company does as many things themselves. Apple make all their own hardware and software, while Dell etc only make hardware, and Microsoft only software. RIM could compete except they only do smart phones (and a flop of a tablet), not computers, or iPods.
Apple is a company that thinks long-term, the road-map for Apple is set for the next 5 years, you could put a monkey in charge, and Apple would do well for that long. However, Steve has hand-picked and tutored all the big-wigs in Apple, they all share his vision and his ethos, he's made a veritable army of Steve's, and the best of them all is Tim Cook, who has literally already done Steve's job twice, both times he's been on extended medical leave.
Wall St. will of course panic, but in a few months we'll all wonder what the fuss was about, not until 2014 or 2015 will we really know if Apple can carry on post-jobs, and even then you're assuming that as Chairman of the Board he'll not continue to contribute the vision and direction of the company.
If I had the money, I'd grab a load of Apple stock when it plummets over the next few days, then sell it in about 3 years, well before Steve's roadmap runs out.
B.