That's a rather selective interpretation. Bearing in mind that TMP was made a decade after TOS ended, at a time the original filming model had been given away, and indeed now hangs in the Smithsonian. Since a new model had to be built anyway, it was decided to make some changes, offer fans something new for the big screen. Compared to some of the concept designs that were done for TMP, the fact they finally went with a design as similar to the original as they did speaks to how well designed the original was. And indeed, most every other ship in Star Trek follows the same basic design pattern, also speaking to how solid a design it is. Not bad for a "painted wooden toy."
