With most platforms that is a valid concern, but with Tumblr you can easily create a local backup your entire blog (http://staff.tumblr.com/post/286303145/tumblr-backup-mac-bet...) as basic HTML, images, assets, theme, etc.

With that said, Tumblr has a very bright future ahead of them.

Sure, but then you're stuck setting up again elsewhere - so why not do it from the start?

Answer to rhetorical question: because we can't be bothered, so we'd rather just pick a provider who is more likely to stick around than not.

That's a totally reasonable consideration but the revenue model of the host platform isn't always the best indicator of its success or longevity.

LiveJournal & Six Apart have longstanding revenue models but aren't hosting users like the President of the United States or large corporate media entities.

Sure, but they're still around. Or are you implying that the President and Corporation, Inc. would bail Tumblr out to keep their blog up?

BTW, that backup app is even less reliable than the rest of Tumblr. I did manage to get a full backup... once.