Knees are dangerous for take down attempts regardless - Barbosa vs Dariush and Batista vs Kishev come to mind immediately and those wern't even to grounded opponents. Knees to grounded opponents continues to change how grapplers approach fighting and for the better.
Soccer kicks/stomps I will somewhat agree with you...but they should still be allowed, even if just for finishing fights against already rocked opponents which is where you usually see them implemented. It's fake and lame that fighters should have to pursue a rocked opponent to the ground and try to finish with punches - risking getting submitted in the process or at least giving the opponent a chance to get back into the fight - when, in the real world, soccer kicks and stomps would finish the opponent.