If anything, people seem more proactive about jumping platforms right now specifically because they learned from the experience of Strikethrough/Boldthrough. For one thing, AO3 already exists and has the bulk of our fanfic safe and sound. For another, technology has accelerated enough that I can just run a few lines of code through my Terminal and HTTrack will download a complete working copy of any blog I ask for that I can then peruse at my leisure completely independent of tumblr's existence. That's waaay better than the mess of an XML file I got when I backed up my LiveJournal!