I absolutely agree. I have one app that I use constantly that hasn't been updated for the iPhone 6/6Plus and higher series of resolutions...
And the app has gone thru dozens of updates since then, like 3+ years. I keep bitching about it in the reviews, they don't give a ****.
Apple forces a lot of stupid things on people & devs, but I'm surprised they haven't forced devs to update for the higher resolution support.
Perhaps with the iPhone X and a completely different aspect ratio and screen format, devs will be pushed to redesign for it and will by the same token update it for the other larger resolutions of the 6/6Plus and higher phones.