iPhone's usable life is limited by irreplaceable battery. This new feature will do nothing to help it.
You actually believe that all iPhones batteries die before apps get updated to newer versions

Because unless you believe that, this change DOES extend the usefulness of old iPhones.
And iPhone batteries ARE replaceable. $79 and Apple installs it for you. Even simpler: third party snap-on battery extender packs are common.
Meanwhile my old iPhone 3G's original battery still gets great battery life... my parents use it. Habits vary of course, and some people's usage will kill batteries quicker than others. To say nothing of iPads: they get such massive battery life that it will take a LONG time before they can't last for a useful session.
When my iPhone battery eventually dies, instead of adding a battery pack or getting the battery replaced, it will live out its life as a stereo component (iTunes library) and wired couch-side email/surf/social media/game station for guests. All the better for having more app downloads

It will be a LONG time before it needs recycling.
Meanwhile, I'm just glad my battery isn't near worthless brand-new out of the box, as it is with my Android friends

They keep buying new Android phones hoping THIS is the one that can last all day and won't freeze up... Guess the Wild West app model (which still doesn't give them a way to have the great apps my iPhone does) isn't working out for them.
If Apple increases the screen size next year the same thing will happen when the iPhone 5 was released last year. All apps would have be be rewritten to take advantage of the new screen resolution. How many developers are going to produce 2 app versions for new releases?
That's not what happened with the iPhone 5 screen change: a single app continues to run on BOTH sizes. Work to be done (sometimes) but no need for 2 versions. In fact, many apps run (fully optimized, no less) on both iPhone and iPad.