I cannot imagine why anyone would risk a longhaul flight without wearing decent flight socks. In particular, why on earth would anyone take medication and not where proper socks? Medication is dangerous - even aspirin - whereas flight socks are just socks.

Yes, of course you should move around regularly and take lots of water. But such things are not always possible or convenient. Even in lie flat bed seats, you may end up in a restricted position. The point is that DVT is extremely serious, and often fatal and is a well known risk of longhaul flying (and trains, for that matter). Oddly, the last statistics I saw (which were some years ago) showed a higher incidence in Business Class than Economy, but this may have been a reflection of business people being more overweight than student backpackers.

Anecdotally, I find that my feet and ankles swell slightly less using them, compared with the times I forgot to pack a clean pair for my return trip. Anyhow, it just seems crazy not to wear them.