The upper mantle is more plastic, but the lower mantle is not solid, merely less plastic. Else deep quakes would not be constrained to current and former plate boundaries. ("Deep quakes", here, are quakes below plates and continents. "Shallow quakes", by contrast, are quakes within plates, plate boundaries, and continents.)
Solids don't deform under stress; they fracture. As the planet cools, it is shrinking. The lack of deep quakes due to shrinkage in areas away from plate boundaries shows that even the lower mantle is plastic. It deforms smoothly despite the shrinkage. The presence of deep quakes at current and former plate boundaries, instead of everywhere, shows that the subducted plate material is the cause of the quakes.
Thus, your argument is incomplete.