This was something that Umberto mentionned in another thread but I think it desserves its own so the idea dosen't "vanishes" from view.
Don't worry, it won't surely vanish, since we always wanted to do something like this but, being compatible with FSX has always hampered it. So, this will eventually make it, but will be a P3D4-only feature and, not just for the pushback: we would like to let you drive any GSX vehicle in first person.
Can't say exactly how much time will take to do it because, as you have noticed, we would have to redo many vehicles with better interiors.