There's a lot more to it than that though, especially if you're targeting normal people and not the HN crowd. What if the subtitles are delayed? What if there are problems with the encoding and there are question marks all over the place? All of this is very solvable, but you can't just say "it's very easy actually, just drag and drop".

I can, because it is. Normal people can do that too. I would argue that VLC is mass-compatible.

Since using your own subtitle file, Netflix just doesn't have to do anything besides displaying them. Finding the correct subtitle is the users job, not Netflix. After all this would be called "use your own subtitle here" and I think Netflix can handle Unicode. Not everything has to be spoonfed for the dumbest user alive. Also, I don't think there is a reliable way to detect delays in the subtitle, especially if it's in a different language then the audio track. If you have a reference subtitle track, this might work, comparing time of first appearance. Any subsequent offset might be due to 25/23,976 which could be detected, but again, this is not Netflix's problem.