The reason for the success of the original design is that it isn't an example of a 1960s design, but it is a good example of a utilitarian design. And this arose from both Roddenberry's instructions of what not to include and Jefferies' own design sensibilities. Jefferies was willing to apply what most people since haven't... essentially, less is more.
This is exactly what Trek production designers these days simply don't seem to understand. Take those DSC season 1 Starfleet ships for example, including the Discoprise. Every one of them is way more over-designed and over-detailed than they needed to be. Because they were designed by someone who feels the need to do that.