OK Jos, I see your point but :
Most musicians prefer 'hardware' to play on, instead of Windows-based stuff.
As stated before some brands try to develop PC-based keyboards/synths, but only very view people have one.
You talk about the low hardware-costs of PC-parts. The 'standard' PC hardware is not good enough for serious musicians. E.g. Hi-Q (DSP) soundcards cost > 500 Euros.
What about the so needed software ?
Imagine that the Native Instruments synths have an average price of 150 Euro's.
What would the price be of a Windows/Linux based 'arranger keyboard' software ? (300 - 600 Euro's ?)
A good quality keyboard (the masterkeyboard) is always needed (500 Euro's ?)
Add these prices to the PC-hardware....
Until now only a very few Virtual-synths on PC impressed me. Most of them sound toy-ish.
Don't forget the PC-concept is still based on IBM 8080 architecture (mid 80's)

Why not Motorola 68000 like Apple uses ?
You could be right in your predictions, but Wersi, Openlabs and Lionstracs did not realy succeed introducing.......
Let's wait and see and in the meantime I stay with my SD1 .... (My grandpa's wise words : "De tied zal't leern mienjong !"
Roel