I like Dave and I respect what he has done for blogging and the community, I also understand his angst here. However I think it is misplaced.
There are two things people do in commerce, one is to innovate/invent and the other is to capture (or extract if you are being cynical) value from that innovation.
Tumblr, like GeoCities before it, have done well on the innovate/invent part and failed at the capture value part. Once at a meeting of west coast venture capital types I heard someone call a business 'making mud pies'. In developing that concept further, they described a business where the founders were having a great time building their technical vision and yet had failed to convince anyone outside of their small circle of friends that it was worth paying money for. Like kids making mud pies or finger painting, the act of creation was the only reward they were getting.
In that sort of situation it's a matter of time before the entity ceases to be.
By all reports Tumblr (like GeoCities before it) was running out of money. If that is true, then Yahoo! has simply saved them from dissolution and retained the traffic (and more importantly the habits of visiting) that folks on the net have built up. They think they extract value out of it, and perhaps they can do so with only a smaller increase in their total expenses than Tumblr existing burn rate. That is a bet they make with themselves.
But let's return to Dave's angst for a minute. Dave saw the deal with Symantec as Symantec getting this great thing they had produced with him to lead it strategically, from Symantec's perspective apparently they saw other things in the deal. Aligning thoughts and visions requires the the guy (or gal) being acquired do three things;
1) They need to understand the real reason they were acquired (was it for people, tech, traffic, brand?)
2) They need to figure out what they want to achieve out of the joined entity (recognizing it may not be possible based on #1)
3) They have to communicate a plan and build a following to insure that what they are trying to do is understood by the organization rather than just one or two people.
Its the strange thing that leaders are often as much controlled by their organizations as they are in control of them. There are few benevolent (or even malevolent) dictatorships, usually there is a group concensus about what is being done. And without that, not a whole lot gets done.
Marissa has stated she doesn't want to 'screw up' this deal. And I am pretty sure she is sincere in that, but it is less up to her than she might like.