It's not irrelevant because people don't want apps, or because of the "quality vs. quantity" argument. People want to buy apps, and they will buy crappy apps just as well as they'll buy quality apps.
The reason it doesn't matter is because people don't care about the investment they've made in any app on their iPhone. Chances are someone who loses their iPhone won't even bother restocking the apps they've bought.
People have bought apps on phones for years. The first "VCast" phone I had on Verizon was the Motorola T720 in 2002. A piece of shit, to be sure, but one that offered apps (BREW, to be specific). It had some kind of yellow pages thing, weather radar, a bunch of games, etc. I think I even bought Snood on it, one of my all time favorite games (thanks to Skip and Ken from Ogilvy for pointing the game out to me).
Do you think anyone gives a rat's ass about throwing away their "app" purchases in order to get a new phone? Hells no... they want the cool new phone. I ditched my T720 like a hot potato for a VX6000, then Samsung, then RAZR, then iPhone, now Droid. Along the way I've probably lost access to dozens of apps that I've paid for.
Even on game consoles, where people invest considerably more in software and peripherals, they're more likely than not to buy another console in the future that is completely incompatible with what they've bought.
No one cares about these apps. It's not like they dropped $150 on Microsoft Word or $600 on Photoshop and then decided to switch to a Mac. There's no training involved. There's no legacy to work within or some kind of value-add. We're talking $5 purchases of stuff that's either self-contained, a minor distraction, or just a front end to a server.
On the iPhone (or Android), a huge percentage of the apps that actually do anything remotely interesting have some kind of "cloud" component. Twitter? Facebook? It doesn't matter what you do on your phone -- the phone apps are just portals into what the server offers. They mean nothing.
So let's stop talking about app store size, or even app stores in general. It's completely irrelevant. Let me know when someone comes up with the equivalent of PageMaker or Photoshop that only works on iPhone, then it will be something worth talking about.
Until then, enjoy your I am T-Pain apps. They're fun, sure, but nothing that would keep someone on the platform ad infinitum.