Honestly , I think developers are going to have a hard time changing peoples behavior. If its vastly cheaper to pay outside of the app store, sure I can see that impacting peoples decisions on how they pay. But, if you're only talking about a couple bucks, most people are just going to say f it, its easier to just use in-app purchases.
Also, if you think about it. people would probably only do that for apps and games that you can play across several different devices.
true, but it requires you to go to their website and pay. You still can't put payment links to third party payment processors in the app. You have to offer something big enough to get them to choose to take the time outside of the app.
You can just embed the paypal payment in a web view inside the app. I currently do that for example for rooted users because many of them have hacked the IAP to auto approve. It's very doable
Also, if you think about it. people would probably only do that for apps and games that you can play across several different devices.