I have always seen this phrase as an optimistic statement by Walt that his park would continue to improve for the better.
I have seen this phrase misused and held up as a defense to support inferior ideas or poor changes. Imagineering could tear down Main Street and replace it with Times Square and tear down the castle and replace it with Twin tower replicas and there would be individuals quoting Walt's phrase.
This phrase supports the ideals of Disneyland that sit at the base of the Main Street flagpole. The phrase supports the words that sit above the entrance tunnels. When one begins to put all of the phrases together, along with Walt's vision of families enjoying attractions together, one begins to see the spirit in which Disneyland was created.
It's easy to see why folks become uneasy when the Disneyland spirit is violated. Using the "never complete" argument has nothing to do with it.
Disneyland is not a museum. I agree. Nor is Disneyland a golf course, a retirement home, a courthouse, or a freeway interchange. I agree. Disneyland is a theme park crafted on a vision.