Using prime lens to zoom

I was doing some experiments with my Nikon 50mm and 28-80mm on my D700. Basically I wanted to find out the movements needed in order to do the zooming with the prime. I laid out all the marks, took numerous shots with both lenses, and measure the distances I moved with the prime to achieve the same coverages from the zoom. Here are my results:

To simulate 24mm - move back 3+ steps (approx 8 feet)
35mm - move back 2 steps (approx 5 feet)
70mm - move forward 1 step (2 feet)
80mm - move forward 1+ step (3 feet)

For a long time I had been itching to get the well-acclaimed 24-70mm f/2.8 lens which has a high price tag of $1887.00. The 50mm f/1.8G I have produces high quality sharp images and it costs $217.00. So with the 50mm I can just move my feet a few steps to get the effect of the zoom and save $1670.00; and have equal, if not better, image quality. All I miss is the convenience.

What do you think, am I missing something?