I also think it is a health risk that  would lead to venous thrombosis

 

It's only a health risk if you're an idiot.  I don't know about you, but many people have desk jobs.  They get up in the morning, sit to eat, sit to watch the news, sit at their desk all day, come home, sit to watch tv, sit at the computer, go to sleep.  No thrombosis there.  They get up during the day to go pee, grab a drink, get some food, walk around, go for a smoke.  

 

For just about three quarters of a day if you can't manage getting up every once and awhile on an airplane to stretch out or do whatever then you deserve to get it.  

...and plus many airlines have video demonstrations on how NOT to be succumed to injuries in-flight.