Excellent piece, Daniel. Concise and authoritative as always!
Re the ‘why people search’, I know the known-item vs exploratory dimension has served us well, but for me it starts to fall a little short when we think about broader information seeking behaviours such as leisure-oriented search or search as learning etc. You could perhaps argue that these belong at a meta-level, in which search activities become ‘subtasks’ serving a greater goal, but they still form a key part of understanding the broader motivational context IMHO and that inevitably affects which evaluation approaches we should consider. I’d be interested to hear how you see these (and other) types of intent fitting into the picture, or indeed whether they can be accommodated by the known-item/exploratory axis -