Then why does design move on, evolve or change about-face? Also how do you know that your design is good, and will stand the test of time? I think what you're witnessing is design that still appeals to your modern tastes. This I attribute more to luck than some superior logic.
"design aspires to a Platonic ideal of simplicity, function and form"
If you believe that than you're throwing out an entire category of design that isn't simple. Some design is incredibly complex ..even messy yet at the time it's considered beautiful and rich because it reflects and invokes human emotions of the time.
Simplicity can stand the test of time because it lacks elements that we associate with the time which can fall out of taste. That doesn't make it better, it just makes it ... simple.
The best design reflects the current tastes of your audience, it does not need to be timeless or even make sense to future generations. Good design evokes an emotional response and that response is dictated by many factors which take into account context.