Does granite become soft as modelling clay at 2000 °C?
Mixtures of different compounds do not usually work like that.  They generally melt at lower temperatures than the constituent pure compounds.  So I would expect your granite to be not like modeling clay, but molten even before getting to the melting point of silica.

Relevant:  https://en.wikipedia.org/wiki/Eutectic_system.

If you want modeling clay, try 1300 °C where the Earth's lithosphere gives way to the asthenosphere.  See the Characteristics section of https://en.wikipedia.org/wiki/Asthenosphere.