Easier said than done. This is trivial for single-purpose apps like Calculator or Weather, where it's easy to predict what the post-launch UI will look like (hint: there's only one screen...)

For much larger apps the "fake screenshot" is harmful. Take the Facebook app for example - what should the "fake screenshot" be? iOS does not differentiate between a completely fresh launch vs. a simple restore from background, and will show the same image no matter what.

So now you're in a situation where you've presented your users with a fake/blank Facebook stream, but really they were restoring to a photo they were looking at. Oops.

Or hell, do you even know if your user's logged in? Would be a shitty experience to show them the fake/blank stream but suddenly pop them back to the signup/login page no? What if they are logged in? Would be a shitty experience to show them the signup/login page and suddenly yank that out from under them.

This whole business is a shitty solution to a shitty problem: apps take forever and a day to launch.