Much like everything else in life, fashion goes through cycles. According to the past, and Vogue, fashion trends tend to repeat themselves every 15 to 20 years. Fashion doesn't just cycle over the years -- it changes and develops during the time of its popularity as well.
Before any of this happens, however, the trend has to make its way past the point of just being a fad. The fashion trend tends to have more consistent enthusiasm over a period of time. A fad will have major popularity over a much shorter stretch of time. Basic products are items that maintain popularity over much longer periods of time and become staples in our everyday life and wardrobe -- for example, t-shirts are a basic product and a staple of wardrobes and fashion collections.
Once sales make it past the point of a fad, fashion will start to develop among the population. The trend will originate with fashion innovators, then are spread by opinion leaders, eventually, the trends catch on and are adopted by the masses. After the peak of popularity for the fashion trend, late adopters will pick it up as the trend drops off. Just before it falls out of favor, laggards will catch on and appreciate its last few moments of glory.
The more features a fashion product includes, the shorter its lifespan. So something like acid washed flared jeans will have much less of a life cycle than a plain black t-shirt. Fashion designers will try to include things with both a short and long life span in their collections, in order to maximize sales before it dies out. The typical fashion cycle lasts about one year, but varies product to product.
15 to 20 years later, lo and behold, the trend reemerges. Although it seems fashion doesn't really go ahead, it comes back and is expressed differently. Each time the trend reemerges, the result is different. According to Italian fashion designers interviewing with Vogue, it has to do with missing experiences of younger generations. Because the younger generations didn't go through the trends, everything seems new to them. Fashion is a very personal experience, everyone needs to experience it on their own, which is why people who have lived through them may want to pretend those trends never existed.
There's a reason we take inspiration from the past, however. People get inspired by former and current icons, whether they be books, movies and celebrities. As long as we look at them and make them our own, there is nothing wrong with taking influence from past fashion trends and ideas.