No economic or political system can be shown to be practically achievable before it's been achieved. If you don't think the following examples are examples of genuine socialism/communism, then that's not an argument against the ideology.

We've had communists fight alongside other leftists. So revolutionary Catalonia was a functioning leftist space, meeting all the criteria to be called communist (classless/moneyless/stateless). It functioned incredibly well for a year before it was invaded.

If you want a longer, but smaller example, Red Vienna existed for about 2 decades and was a fully functional socialist space that improved worker's lives before being outlawed by a regime change.

If your position is that imperialist capitalist nations will always invade/outlaw well-functioning socialist/communist systems, you can't know that for sure, but it's definitely a possibility. That doesn't mean the entire idea is worth throwing away.