If your PC "box" is made up of straight lines then no matter what angle you view it from, those lines will ALWAYS appear as straight lines. Perspective may make a line look longer or shorter and may make a line look tilted, but it will never make a straight line look like a curved line.
A straight line, viewed from any angle, is always a straight line.
A curve on the other hand is different. A curve will ALWAYS appear as a curve EXCEPT when viewed anywhere directly level with the plane of the curve. When you view on the plane, the curve appears as a straight line. So look at the rim of your coffee cup and it's always a curve unless you look at it directly level.
But when you look at the top of that cup directly level, then look at the bottom of the cup and you have to see and understand that the bottom must look curved because your sight line is still level with the top of the cup.
With your PC front, there is only ONE position you can view it and actually see a perfect rectangle and that's when viewing it dead centre. Move up or down and as your diagrams suggested, the rectangle becomes some kind of trapezoid.
This is useful discussion TBH. If you were to take up art and try to draw a tall glass you'd quickly see that it looked completely wrong if the side view was painted as a rectangle :-)