The issue is that a 2GB RAM device will need to be upgraded sooner than one using 4GB. Although iOS is designed to run at lower RAM levels -- not all apps that run on iOS do. Less RAM eventually means less value.
While this is technically true, as a developer myself, I have a hard time abandoning older hardware. If I support iOS 8 and 9, then I have to support the iPad 2, 3, 4, the Air (most RAM constrained outside the 2), and the Air 2. The users of those devices all outnumber iPad Pro users. Do I just say "no Pro, no play"? Of course not. Just like I won't exclude users on a MacBook Air, just because the MacBook Pros ship with more RAM and more CPU power. But I can do things like what Procreate does, and place reasonable limits based on what the user has if it makes sense to do so.
And as I've pointed out in other threads, a chunk of that RAM is there to feed the larger display. And the impact is bigger than people think. Since each piece of an app's UI has a pixel buffer, and now some of those buffers are all bigger (except maybe icons), or more numerous (more visible table rows). And those buffers are cached for performance reeasons.