One of their most valuable assets is Android Yahoo Mail app, that they are methodically killing: my mom (a loyal Yahoo user of 18+ years) says it has become too complicated (grouping conversations), with an unpredictable spam filter. Until recently it suffered from a serious memory leak (which was not fixed for years).

They already successfully killed the Android Yahoo Messenger app (remember when Yahoo Messenger was huge on desktop?). Ironically, WhatsApp founders are from Yahoo...