Three things which would be a major issue for Yahoo! on that sort of search would perhaps be first that they themselves rely so heavily on content syndication to power their various verticals, second they keep losing search market share (especially as more search happens on mobile devices and Google has mobile locked down with their Android contracts), and they also screwed up their old directory before they moved it to Yahoo! small business as part of the Alibaba share spinco.
I also don't see how Yahoo would effectively differentiate their search engine enough to be able to (profitably) buy share at prices set by Google, particularly if they over-promote their internal results & rely on a smaller search index.