I'd rather have 326PPI pixel perfect display, than having 3x density downscaled to 2.7x. Also, you can't just no-brain bump the pixel density without it affecting the UI elements' size.
Downsampling is actually extremely effective, and in many ways is better than native. That's why a lot of modern PC games feature this. e.g. A 4K-rendered video game downsampled to fit a 1080p display can look quite a bit better than the regular 1080p-rendered content. Definitely more resource intensive than regular 1080p, though. That's likely why 1080p on the 6 Plus looks noticeably sharper than 1080p on the smaller HTC One M8.