There should be a trial flight arranged by passengers to test out the practicality of the advice. Take an Air Canada 777 flight with 307 economy passengers and get everyone to move around the economy cabin every 30 minutes or so. I suspect even staggered timings wouldn’t work.
According to NHS Choices, DVT from long haul flying was first described in 1956 so it’s not exactly a new condition. The prevalence of tighter seating in all directions in today’s ‘modern’ aircraft is surely the cause of the rise of incidents, and therefore publicity and public awareness of the condition.