Alex...
In order for new products to sell, they must at least seem to be new and improved. The old Hoover Citations, the Royal metal uprights, and the Sanitaires all use simple technology that can be tweaked, but not significantly altered, or its effectiveness diminishes. Why would someone buy something newer if it looks like the old one they already have?
The consumer preference for lighter, easier, faster has been discussed to death on this forum. It has to have style and convenience appeal to sell, and sell it must in order to stay in business. The newer vacuums must use smaller high-speed motors to accommodate the trend. These of necessity burn out sooner due to their speed, composition and construction. Effectiveness depends on a confluence of factors involving design, application of the product, and user competence, again discussed to death here. Performance is not achieved by good design nowadays, but by the application of brute force. The trend is toward ever shorter life cycles of product. It is what people have been trained, coached, encouraged and even bribed to want. So it is what we have.