A product cycle of two years isn't anything new for Yamaha, and other manufacturers changed their cycles as well.

The PSR-7000 (1995) was only two years the junior of the PSR-8000 (1997)...the CVP series used a two year cycle from 1991 to 1997, for example.

Perhaps you could investigate what was happening with the competitors at those times, and maybe there may be a reason.

Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.