Re. Trickle Down technology
Shimano does 'trickle' the features from the top groups down to lower groups through the years, - features, but not quality. The materials and moving parts of DA and Ultegra are better quality than 105, 105 is better than Tiagra… etc. Yes, you can get a very affordable 9 sp set of parts, but they will never be quite as nice, nor will they last as long or work as well over time as, for instance, 9 speed Ultegra from 2001.