It may be that the people who buy Tyros in the UK (and this is a big home user market) have the money available and are willing to spend it on something they enjoy, after a couple of years they are ready for change. It would be a nightmare trying to get them to do a Ketron type upgrade as they are mostly in their late 70s and up. Changing after two years keeps them loyal to Yamaha, otherwise they run the risk of losing customers to the competition.