Whenever Yahoo! comes into discussion, no one talks about the other side of Marissa Mayer, the former CEO who fucked up Yahoo so bad. I had a friend back then who used to tell me how toxic the work culture had become in there. She apparently openly hated men and discriminated against them in the name of empowerment (and subsequently lawsuits were even filed). She was a glorified senior manager from Google basically who had no idea how to run a company. The Tumblr acquisition was proof. Most of the employees had lost faith in her after that acquistion basically. She, on the other hand, took away millions home while everyone who had invested their sweat equity and trusted her even one bit got screwed royally.

I don't know the finer details of that, but weren't they already travelling towards obscurity by the time she arrived?

Yahoo was a defacto entry point into the wider web and as Google gained popularity I think the average web savviness of people meant people had less reasons to arrive at Yahoo. Having a search deal with Google at least guaranteed their relevance for a while.