Communism is not a bad economic system. It is not an economic system.

Every economic system is fundamentally an allocation algorithm. Somehow it must be decided what is produced, and who can use/consume that. An economic system is the “recipe” (algorithm) for making this decision. If you specify such an algorithm, you can make statements about the result.

For example, Capitalism:

1. Start Sate: Everyone starts with whatever they "own" and is free to decide
what do with it (including their time).
2. Everyone has the option to trade if both parties agree. Continue trading
until no such trades exist anymore.

You can then show that under specific circumstances the result of this algorithm is pareto optimal.

pareto optimal does not mean perfect in any sense. Just not “obviously sub-optimal”

The problem with Communism is, that it is an under-specified algorithm. You only state that everyone owns everything. But the question who gets to use/consume these things at any time, and who should work on what thing is not specified. So this “planning” still has to be done somehow.

And since there is no mechanism which would do this decentrally like in Capitalism, it usually ends up being a centrally planned economy. And that requires a planner. The issue here is, that a group of people will never agree on what is best to do for the group. Even if the entire group is completely altruistic. The reason for this is, that no-one knows how other people feel about certain things.

Example

Consider a post-scarcity society of Alice, Bob and Charlie. They have housing, food and all the basic necessities. Now they have to decide what to do with their time.

• Alice is very curious and wants to explore space. So she wants the group to build a spaceship.
• Bob likes to relax, so he wants the group to build a swimming pool.
• Charlie is philosophically minded and wants the group to discuss the meaning of life.

Then what should the group do? None of them is egoistical, Alice and bob would be happy to share the spaceship/pool with the group. The issue is that they simply do not want the same things. But if one of them would be the planner, then they would simply decide to do whatever they want to do and impose it on everyone else. You could of course vote on this issue,

A voting as a mechanism to generate a public preference relation is deeply problematic (see Condorcet Paradox)

but even if you manage to get a majority for something, it would still result in the majority imposing their will on a minority. Essentially forcing them to work on something they are not interested in.

So you might think, that everyone should just work on their own project. But this is a very capitalistic outlook on things already. Since you essentially say: Everyone owns their time and they can do whatever they want to do with it. If you then introduce different skill sets, then it might stop making sense, that people work on their own projects. And suddenly you would have to introduce some form of “exchange system”. So you would likely end up with the second part of capitalism as well.

Summary

Communism is not an economic system, because it does not specify how people should use resources. And this vacuum is usually filled with a planned economy which is inherently authoritarian to some degree, since the priority list of the planner will never coincide with the priority list of every individual. Therefore it imposes priorities on other people.

In some cases, this imposing of priorities might be acceptable. Most people would probably agree that enough food and housing for everyone should be top priority. So you can artificially move these things up the priority list (generated decentrally by the capitalism algorithm), by guaranteeing them with unemployment help or universal basic income.

China ultimately decided to use the “Capitalism Algorithm” for most of its economy, letting it deal with the details. But they then modify the result by heavily subsidizing certain sectors. Essentially planning certain things centrally, while leaving the details to the capitalism algorithm.

TL;DR: Economists do not discuss Communism, because there is no system to discuss.