I agree with MidtownKid's characterization of the Plaza. What is being discounted here is that what was created is a very walkable environment with mixed uses that is very integrated into its' surrounding neighborhoods. The reason the Plaza is significant, and why so many architects and city planners study it, has to do with the level of detail in its' design; and the level of planning and quality. It's because it's more than a shopping center. Nichols sought to include art, landscaping, and numerous examples of eye-candy for the pedestrian. It's the details. It's cohesive, but not sterile. He paid attention to "layers," which is what a lot of modern developers fail to do.
He knew that parking would become important in modern merchandising, but he sought to minimize its' impact by planning to eventually hide most of the parking. However, the Plaza still does have a lot of street parking.
It's less important that he sought to mimic the architecture of old-Europe, but that he wanted to copy what made old European city planning so effective--and long-lasting. The details and texture of the environment. He couldn't recreate European cities in the American environment, but he took elements of them and applied it to development realities. He was patient, and knew it would take time to build this type of environment over the long-term, which is what happened. The Plaza has not been a flash-in-the-pan development. It struggled at first, but he kept to the original plan.
Even in the residential neighborhoods he developed, Nichols didn't just build cookie-cutter rows of houses that were thrown up quickly. He mixed architectural styles so that no two streets were alike, and that the housing wasn't monotonous. He knew his customers would seek variety, so he offered a lot of different housing types so everyone could find something they liked. The proof of his success is that very few of the exteriors of his houses have been modified over time--or modernized to appeal to fleeting tastes.
The thing that should be acknowledged is the long-term success of the Plaza. Many other retail centers have been built after the Plaza, and many of them are struggling, or have even been demolished. Nichols' residential neighborhoods are approaching 100 years old. They are still in good shape, and highly sought after. Few neighborhoods in Kansas City have been around that long and remained stable, and intact neighborhoods. He also taught the lesson about the importance of building maintenance, and keeping up the appearance of his properties--to maintain property values and rents.
It's not just the Plaza. many of Nichols' other retail nodes still are viable.
The racist practices are indeed a blot on his reputation, and they did have a long-term effect on the City. However, as was noted earlier, there were probably other builders who were as equally racist, who are responsible for much of the built environment of this country.
When it comes to that retail strip itself, the significance of it often has less to do with who the developer was, and instead more to do with who the architect was, or in many cases, things that happened inside the building that makes it historic.
The retail strip itself is not that significant for its' design or history. It's a pretty basic building. Should it be demolished for a larger-scale structure, it probably won't create a lot of opposition as long as there is a replacement structure that is denser, and better designed. If it's just demolished for a surface parking lot, then there is an argument to be made to save it simply because it provides retail space to the neighborhood.
Oddly, that retail strip is probably the most plain, non-descript building that Nichols ever constructed.
If Nichols were alive today, I would say that he would have continued to make the area around the Plaza denser--probably adding more apartment buildings, and even expanding elements of the Plaza environment to nearby neighborhoods like around Cleaver and Troost, and Paseo; south along Main; and even further north along Roanoke Parkway, Belleview, Madison, Main, and Broadway. I think he would have continued to make architecture the focus of his developments.