I'm pretty sure that we'll have some kind of service or API to convert voice to text soon, as AI engines progress. WebVTT spec as I understand is just for showing captions in the standard HTML5 video element, which is handy for standardized playback but doesn't help with recognition in any way. Still need something to "understand" all the talking.
Perhaps that's an idea for a community? Say, people share most interesting/popular videos and some members write up transcripts for the rest of the community to read. You can monetize by setting up a marketplace for quick transctipt service, or tipping, or subscription based access with revenue sharing.