Historically, a new PlayStation has been absolutely impossible to get on launch day if you didn't pre-order it. Hell, it was hard to get one within several months of initial release.
Really, I think that's one of the primary factors many people overlook when appraising previous hardware launches. People can point to unimpressive software lineups right out of the gate (although those arguments don't have quite as much merit as some believe). The bottom line is that if no store in existence has the damn thing to sell, then what difference does it make? No stock, excitement starts to dwindle fast.
Recent reports for the PlayStation 4 are encouraging, though- it seems Sony is primed and ready to flood the market with lots of systems. Some reports claim we could see as many as 10-15 million units around the world in 2013 alone, which is pretty damn good. Factor in what Sony is claiming to be their "best launch lineup of games ever," and you're looking at a product that could fly off shelves. Above all else, though, stores need the PS4 to sell, regardless of consumer interest. Love it all you want; if it isn't there, it can't be sold. You have to tempt the consumer sometimes, you know?
With more straightforward architecture and cheaper components, there's really no reason why the PS4 can't be manufactured with relative ease. And while it was always kinda fun to feel somewhat privileged on launch day, knowing you were one of a very small number of gamers who brought home the new system…well, it's just not good from a business standpoint. Oh, and it should also eliminate other nasty occurrences, such as the news of muggings and thefts that occurred at the launch of the PS3 (and PS2). We can really do without those, too.