It's not so silly when your culture is situated right on the Pacific "Ring of Fire" as is the case for Japan. In times past, you wanted to make your houses light, so that if there is an earthquake, less stuff falls on you and your family.
Imagine what it would be like if the entire culture was constantly metaphorically whispering to you from the moment of your birth about the temporary nature of life? Sorry, but while you can tell yourself you're doing it, you can't actually do it -- your imagination isn't that good! (Otherwise, there would be far fewer systemic social problems.)
In Japan, there is no distinction between high and low art. (This is absolutely related to the rest of this comment!)
Are you implying that people think the world ends when they die? If I'm going to do useful work, why not do useful work that will help out people even after I'm dead? Right now I'm living in a building that was built in the 1880s. I'm happy that they built it to last.
Non-sequitur!
Ps. Not to mention natural disasters and climate change, either modern or historic ala Maya or Saharan changes, for example.
So, yes, design with impermanence in mind.
That seems to be some very silly reasoning. Should we build our roads and bridges to be impermanent, because life is? I'm not arguing with your first sentence, about it not being surprising that unmaintained structures tend to deteriorate.