Wait, you're saying if I can control that URL I can prevent all iOS devices in the world from connecting to the Internet ? That sounds seriously dangerous. Or am I mis-understanding?
If you can control that URL, you can probably control ANY URL, and thus you can already prevent any (not just iOS) devices from connecting to the internet _via your wifi ap_. No surprises there.
iOS falls back to 3G when the page doesn't respond as expected.
In fact, if you control that URL, then you are operating a captive network, which is precisely what accessing that URL is meant to detect.
What if the URL gets successfully DDOS'd?
Because your botnet will generate more traffic than every iOS device in the world connecting to a wifi network?
Every iOS device connects at the same time?
There's a large enough proportion of them connecting to wifi networks at a given time and a very large number of devices. Plus it's a very small static page.