Thank you for your replies.
I think a reason why newer is not always better is that we never see people who design a keyboard and make decisions, we just know a name of a brand. But there are actual people and over time some people come in and some go away. So, for example, PSR-S910 can represent initiative of one person, and PSR-S970 represents some disagreement of other one.
While Yamaha’s progress is still rather obvious to me, looking at Casio it seems like they are going up and down. They had something like MZ-2000, then nothing like that. Then they had some luck with WK-3300/3800, but the successors were rather questionable.
Another reason is that we actually don’t expect to pay more for today’s keyboards than we paid for previous ones. I don’t know about other counties, but in Russia prices for food and services tend to go higher and higher, still, for electronics it’s not that obvious. I don’t know a lot about economy, but I expect that Yamaha has to rise payments for its workers as time goes by, and yet it has to sell Genos for the price of Tyros 1-5, yet to make it better. Can it be a reason why they eliminate what they consider “unnecessary”, well, knobs with no marks on Genos speak for themselves.
Apart from this, there’s surely a pressure from society, just as on society. Buying things has become sort of religion, dependency or addiction. We expect it to make use happier. Most obvious it is seen if we look at iPhone industry. I know people here in Russia who don’t really have a stable job or an idea of what they are going to do next, but buying a recent iPhone is a necessity for them. Then they take selfie with it and you know what? It doesn’t look pretty.
I think Yamaha has accepted rules of this game earlier than other manufactures, I’m sure that Yamaha has a rather aggressive team of marketers, and in some way they want to be Coca-Cola or Macdonald’s of a musical world. So it’s no longer like buying a bicycle of a drill in order to use it forever, it’s like renewing you subscription to prolong your happiness, and to do it you have to buy a new keyboard.
Meanwhile, apart from professionals and enthusiasts, for majority of people a musical keyboards is just something that they might use from time to time, not even knowing what model it is and they are unlikely to buy more than two or three during their lifetime.