A planned economy has been tried. However, even after removing all market mechanisms, corruption amongst the party managed to shatter the union.
As much as Deng is hated, he managed to set the foundation of a mixed economy, which, as we can see by the great growth of China, was probably the correct thing to do.
Later adaptations by the CCP managed to incorporate precautions to prevent internal corruptions, and furthermore, bourgeoisie corruption to avoid the dissolution of China and shock therapy.
Furthermore, after the dissolution of the USSR, many communist parties followed China in pursue of a mixed economy, leading to growth and the betterment of society as a whole.
That’s not to say that China doesn’t have its flaws. Particularly in its neutrality and lack of action internationally.
The problem with the Soviet economy wasn’t simply because it was planned, but because the way it was planned it led to a very centralized command structure with little flexibility, accountability and transparency.
I’d rather not speak what would be the ideal system, but I rather argument in favor of adjusting the economic management through trial and error, in a scientific way. And that market should be gradually replaced, and the way planning is made needs to be revised, to avoid inefficiencies and concentrations of power.
By the way, China’s mixed economy does not exist simply because of efficiency reasons, but because it chose to open the market for foreign investors. Because of this, China couldn’t exercise control anymore over its own industries, but the CPC used this opportunity to close the technology gap between their economy and the West.