tl;dr: updates are real expensive, the TA app violates some of Apple's new App Store guidelines involving not having apps that recommend other apps, we've had minor updates rejected, iPhone 6 support would be a pretty big rework and likely would get rejected as well. TA app will likely exist in the state its in until an iOS update totally breaks it, at which case, we'll see.
Maybe this is more obvious on a 6+, but on my iPhone 6, the test and pictures all look super-crisp. I don't know what would change or if I'd even notice if it became iPhone 6 optimised.
I forgot to mention, I was given TA as a suggestion when I was selecting options intern NEWS app in iOS 9. It seems that don't care if you do what you do, as longs as its not on their App Store.
I would really love an update. On 6+ it looks and feels very bad. Everything is BIG and somehow blurry (zoomed), spoilers aren't readable, you can't "swipe", and pictures are only closable through a button too. The experience was good a few years ago but now it feels totally sub-par. And on a big phone it's totally crap. If I would be new to this page, I won't use the app anymore...
