Yes, but if the difficulty increases every two weeks, then wouldn't people with AMD cards hit a wall eventually too?
Yes, but the Radeons will hit the unprofitability wall at a much later time than the Nvidias will. Heck, depending on how much you're paying for electricity, most Nvidia cards are probably already unprofitable, whereas Radeons still pay to keep themselves running (and then some).
There's only one Bitcoin network. If you aren't using the most efficient hardware, you're going to lose a lot faster than people using the proper hardware will. Your inefficient Nvidia rig (as measured in MHash/KW-hr) simply can't keep up.