Neither of those are what leftists say. Capitalism doesn't work because of the structure itself, you have problems like the Tendency for the Rate of Profit to Fall, and the inherent exploitation within. You cannot have Capitalism without exploitation, and you can't have Capitalism with democratization of production, even if you had a perfectly selfless Capitalist, it still wouldn't be democratic and would still have the same structural issues.

Similarly, Communism isn't "people working for the common good," it's people working to improve their own material conditions. Just because production is democratized doesn't mean it depends on people working for absolutely no reason.

There are non-strawman arguments you could make, but this ain't it.

"Capitalism does not work because people are selfish, and selfish people are incentivized to harm their fellow man by capitalist structures. Under socialism, selfish people will work toward the common good because working toward the common good is the easiest way to earn recognition and status"

"People are selfish, and it is in 99 percent of peoples self interest to overthrow capitalism in order to improve their material conditions"

I don't think most people are selfish to the point of it being harmful. I think the problem is that a small number of people are, and those are the people who are in charge of things, where their selfishness can do way more harm.

As others have mentioned, though, a lot of behavior is heavily influenced by the incentive structures people live within. This can apply in very obvious ways: for example, when trying to get from point A to point B, people will use the mode of transportation that makes the most sense for that trip, which is heavily dependent on the infrastructure that exists between those two places, and that's why the Dutch will bike five miles, the Spanish will catch a train across the whole country, and people in Houston will drive across the street. It can also apply in more subtle ways, though, and that's where capitalism comes in. To pick one example, companies that are owned by their workers are more stable and better places to work than traditional privately owned or shareholder-owned companies, but it goes far deeper and gets far more complex than that, too.

People are responsive to economic incentives. If the incentives favor doing good things, then good things happen. Otherwise, you get what we have now.