A fully planned system has also shown to become highly inefficient

Nobody was arguing that.

I don’t think you understand what actually is meant by the term “capitalism”. Capitalism does not mean free markets. Capitalism primarily means the ownership of the means of production in private hands. You can come up with a system which is highly regulated, to some degree even planned, which can still be considered capitalistic.

On the other hand, it is easy to imagine a socialist system whose economy consist solely of companies fully owned by the people that work there, i.e. the workers, while the companies themselves engage in a competitive and free market. It would be just like today, except workers have a say in who leads the organization, and how, in a democratic process.

In short capitalism != free market and vice versa.