No one is going to write an iPhone 5 (fifth gen hardware) app? You're just like the people who thought "No one is going to write retina display apps, most people have the old phones." Now if you don't support the retina display, your app is very much inferior.
There will MOST DEFINITELY be a good amount of fifth gen only apps, and even more 4th+5th generation only apps. Just like some of today's apps only support the 4 and 3GS.
There's a huge difference between SUPPORTING the a-5 (or retina display) and REQUIRING.
Look, I'm not against the A-5. Whatever. Knock yourselves out with it. I'm just saying that when push comes to shove, you're going to wish you had LTE compatability more than an A-5 processor.
All the processing speed in the world can't save you if your phone can't access content fast enough -- and with the move to cloud-based storage on the horizon, anyone carrying a 3G phone is going to have a miserable experience.