EDIT: I just realized we might have crossed-wires, I was referring to the loss of Garry Tan and not the loss of Brett Gibson.
Obviously you have more knowledge of Posterous than I do, but generally a founder leaving tends to have negative implications (it scares potential investors, damages momentum).
I use both Tumblr and Posterous and from a user perspective when Posterous pivoted to be focused on "spaces" it seemed like the decision was panic driven. The reason that Tumblr was succeeding over Posterous wasn't due to the feature set of Tumblr but rather the community that Tumblr had.
Posterous had advantages over Tumblr and in many ways it's the better blogging platform, but rather than playing on those strengths and improving them Posterous instead went after the social aspects that Tumblr was succeeding in without fully appreciating that it couldn't just take a social feature set and foist them onto it's user-base.
That's a very different statement than saying someone's departure put you on the path to success. The way you originally stated it implied fairly heavily that the departed founder was a negative influence holding you back.
