In that case the effect on YouTube's bottom line would be even smaller, if any. Creators posting their videos on other platforms would have a negligible to no effect on YouTube, given that it would remain the default (read "only") video platform for the overwhelming majority of the viewing public. A creator announcing "You may also see my videos on DailyMotion/Vimeo/Twitch/PeerTube/wherever" would simply be irrelevant to their mass audience, who would react anywhere between "Ok, whatever" to "I don't know what that means, ignore".