This isn't about if Michael never Moonwalked, it's about the possibility of the Motown 25 being the first and last time he did it.
Michael's dance was more than street mixed with classic influences. He already developed a unique style of his own by 'Off The Wall'. Just look at the breakdown for the 'Don't Stop 'Til You Get Enough' performance in the Spike Lee documentary.
I don't agree that Whitney and Prince faded away with time. They were always remembered as legends in their own right, regardless of whether they sustained commercial success.
The Moonwalk didn't define Michael's artistry. He had many other aspects such as his short films (which he pioneered) and song writing.
Michael didn't do the Moonwalk during the 'This Is It' rehearsals. Yet the dancers went wild. I wonder if the reaction would have been the same from the audience, had the concerts happened and he omitted it completely? He once said that the fans wanted him to do it in a specific place each time, but he wanted to change it up. I think he improved it each time. However, the first will always be more special because no one was expecting it, and because of the perfect way he set it up: the timing, the intricate wrist movement, both feet facing in the same direction, the big spin afterward.. I don't think any of that has been topped!