I don't think you're right about Twitter. Twitter is a real time information company, more or less. No journalists or news savvy people to to Tumblr for content. There is value in having live content coming from "content creators" than in cases like Tumblr where most of the content is not original at all. Twitter doesn't know it's own value because it's poorly run, but I don't think they ever wanted to be like Tumblr.
I think they want to be more appealing to casuals and are willing to sacrifice their core users at the altar of a timeline that is not a simple reverse chronological but tries to guess what you likely like reading.
