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".

Just because they can collect revenue from other platforms doesn’t mean it will be sufficient or anywhere near what they were getting at YouTube.

Of course not, but it is a hedge against any decrease in revenue from YouTube, as speculated here. A hedge that doesn't cost anything.

The act of publishing on PeerTube together with Youtube (or even one day earlier!) can be a good protest already in itself.