Can my web browser play FLAC or Opus out of the box? Mind, I'm not going to tell you what my web browser is. Because that's why they picked MP3: if you're using MP3 you can confidently say "yes" because it's a computer in 2018.
Flac isn't really for browser playback, but for lossless backup. Normal browsers should play Opus fine today. Some crippled ones might not, but for that there are projects like this as fallback:
https://github.com/brion/ogv.js/
So I see no reason not to use Flac and Opus for music services.
Kind of selfish of you to think Flac should be a standard where some people wouldn't even know what it is.
People don't need to know what it is by default. But they should be able to make lossless backup if they need to.