Pre, to avoid, as far as possible, failure to understand the basis on which each of us is mustering his (/her?) thoughts, perhaps you would give an “agree/disagree” answer to each of the following:

1. If convection were to occur in the mantle, it would not be straightforward Rayleigh–Bénard convection.

2. Mantle tomography indicates that the core-mantle boundary is relatively isothermal, homogeneous and stress free.

3. The upper mantle boundary is heterogeneous, stressful and subject to varied and fluctuating temperatures.

4. Convection can be driven by bottom heating, top cooling, or even side cooling.

There never was nothing.