We do see the same in physical engineering too. At some point some products have plateaued, there's no more development. But still you need to sell more, the designers need to get paid and they are used as status symbols and so on.

One example is skirt length. You have fashion and the only thing about it is change. If everybody's wearing short skirts, then longer skirts will need to be launched in fashion magazines and manufactured and sent to shops in order to sell more. The actual products have not functionally changed in centuries.

Or people just start ignoring the trends and only replace their clothes when they wear out.

I don't think that fashion trends are comparable. I think that fashion trends are fine in concept–things get old and we switch things up. It's the way the human superorganism is able to evolve new ideas. Unfortunately, capitalism accelerates these changes to an unreasonable pace, but even in Star Trek communism, people get bored. The cultural energy that birthed one style is no longer present, we always need something new that appeals to the current time.

But clothes still have to look nice. Fashion designers have a motivation to make clothes that serve their purpose elegantly. Inelegance would be adding metal rails to a skirt so that you could extend its length at will. Sure, the new object has a new function, and its designer might feel clever, but it is uglier. But ugly software and beautiful software often look the same. So software trends end up being ugly, because no one involved had an eye for beauty.