"It's a pity old 5700 had a better piano voice!!!!. Why Yamaha make now a worse piano voice? "

I believe it had something to do with the design of the new keyboard's sampling technique and sound engine. The same seems to hold true for the vibes voice as well.
The 5700 had fantastic vibes, and in the hands of a skilled player, it was difficult to detect any difference between the 5700 and the real McCoy.

Gary