Tumblr users will systematically migrate to WordPress.
Amend that to "remaining Tumblr users" and, "sure, maybe".
That said, if you were someone looking for a WordPress style experience, you were previously already using WordPress, not Tumblr. Tumblr really excelled at "stream of consciousness" type of content. It filled a nice spot between FaceBook-style sites and Twitter-style sites without forcing people into the straightjacket that is Wordpress (and similar CMSes).
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