Maybe, just maybe if the entire design world hadn’t switched to pure white backgrounds, nav bars, status bars, tab bars and everything else, then maybe we wouldn't need a dark mode.
And not being the default means dark mode will be likely treated as a second class citizen.
Related: Please stop making apps with pure white everywhere. This is doubly true for anything displaying photos or video. Photos.app for macOS, I am looking at you.
Hopefully one day Sketch gets there too.
My bet is Apple will reveal dark mode when they release an OLED iPhone. That way, the backgrounds will blend into the device screen itself, like with the Apple Watch.