It doesn't even have to be more efficient, necessarily, just valuable enough to be more worthwhile.
In this case, they seem to be saying long-form archives aren't helping their business and are very expensive.
Of course, since that also de facto means people start pointing to their YouTube pages as their content archives, that means they think they have such a better platform for live content that they can survive people doing the calculus of "well, if I have to host my old content on YT anyway, why am I using Twitch if I'm just going to upload to YT after..."
Whether that's true or not, we'll see. (One might argue this is a given comparing the number of people I know who stream on Twitch versus YT, but Twitch is also the place that thought people wanted them to integrate a game store in their desktop app, and appears to have the attention span of a squirrel in long-term platform initiatives, so...we'll see.)
(I work for Google, I've never worked on anything related to YouTube, opinions my own.)