My wife would like this added to the conversation:

If you booked a car service to take you from A to B and the journey was 90 minutes, you wouldn't put your feet up on the dashboard of the car, even if your knees were sore. Why wouldn't you do that? Out of respect for other people's property and an understanding other people pay to use the same car and expect it to be kept to a good standard.

If you did put your feet up on the dash, I'm sure we'd all agree, the driver would be well within his rights to ask you to remove them.

Bus companies and train operators put up signs asking people not to put their feet on seats. Why? Because some people are too self absorbed / inconsiderate to understand this is something you simply should not do.

I completely fail to see why anybody would think it was ok on an aircraft.

This isn't snobbery, it's about - as others have said - respect.