There's plenty of room for a normal human being to sit like a normal human being. Unless you are 350lbs, 6'5" or flying Spirit, there is enough room to sit in a seat as it was designed. There's also the issue of clear air turbulence to consider. Many of those more advanced positions require people to unbuckle their seat belt, making them much more likely to be injured in the event of significant turbulence. As for people putting their feet on places where they don't belong, A- it's incredibly unsanitary and B- It's impolite to intrude on the limited space other people have in the confined quarters of an airplane. The people with their feet on the trays and armrests in front of them are usually the same people that go to the lavatory on the airplane without shoes too.