I'm not saying the Nexus is bad, but just look at the sales. Apple will not be surpassed in this market any time soon.
Apple is being surpassed, but you won't see everyone switch from the iPad or iOS overnight. The iPad and iPhone were the first big products in capacitive touch tablets and smartphones, respectively, and it's hard for most users with those products in their possession to switch.
The switch is starting to happen with some users, though, and the longer Apple lags behind on hardware or pace of OS/software updates, the more users will switch to Android devices. This will happen each time there is a delay. They can't just catch up to Android, they need to leapfrog them when the next iPad mini comes out.
iOS 7 should make things steadier for iOS, but if everyone isn't satisfied with it right out of the gate, we could see the first major exodus from one platform to another. And by "major" I mean a few percentage points out of total market share, mostly conversions of consumers who were waiting on iOS 7 before choosing a new device to upgrade to. There's never going to be an overnight switch of people to or from one ecosystem to another, not without a really colossal failure like an update bricking millions of devices (not very likely to happen).
The Nexus 7 doesn't have enough marketing behind it yet. I think word of mouth can increase sales, but not enough people are aware of how good it is. I think there are still plenty of average consumers out there whose last experience on an Android device was pre-Jellybean, and unfortunately that has colored their opinion of Android. We're still seeing people talk about who laggy Android is - even on tech forums - when that's just not the case on 4.1+ devices with stock Android or minimalist custom UIs. Some of the Apple users are coming around, though, when they get their hands on the 2013 Nexus. There needs to be a "This Is Android" campaign where ASUS/Google lets people demo units in various cities.