Yahoo search is a thin layer on top of Bing these days. Yahoo only ran it's own search (crawling etc) about 2003-2009, all other years it was outsourced (to Inktomi, Overture, Google, Bing)
http://news.softpedia.com/news/Yahoo-Sells-Search-Business-t... (2009)
I'd say after the Microsoft deal there was no turning back. Too many people, machines, IP moved. Overture bought Inktomi, Altavista, alltheweb search engines. Yahoo bought overture for 4 billion. That was 100s of engineers. Starting again would be an (in my opinion) impossible hiring task, 10miles away from the Google Campus who tend to make excellent counter offers. And at web-scale you cannot be great unless you index, well, almost all the web.
