Your thesis is correct (single purpose app can survive in a world full of multi-featured competitors by being insanely good at one thing). But I disagree with your interpretation. Evernote is THE multi-featured competitor in notetaking arena. It does too many things, each of them in mediocre manner. Then comes all the "single-purpose apps that are insanely good at one thing only" which disrupt Evernote. Think of Instapaper (Saving webpages), Simplenote (Text syncing), All kinds of TODO apps, they all are good at one thing and do it well. That's why Evernote is not doing well.