What would happen if the earth's mantle was completely solid? Why do you think so?

The earth's mantle is comprised of silicate rocks that are usually in the form of liquid. Its thickness is nearly 2,900 kilometres, forming about 85% of the total volume of the Earth. One of the main significance of this layer is the formation of the convection current, that initiates due to the heat energy provided from the interior of the earth, as a result of which the lithospheric plates above the layer of asthenosphere move from one place to another with respect to a fixed pole.

But, if the earth's mantle was entirely solid in nature then these heat energy from the core of the earth would not be sufficient to form these convection currents, due to which the plates would have not moved and there would have existed only a large super-continent and a huge water body.