Yuck. I am not big on Marx, but I think he gets a lot right in his Critique of the Gotha Programme. Social democracy is more right-wing than the liberalism it purports to replace. Its intention is to maintain capitalist norms by bribing the poor with their own labor-product and using the capitalist state as the mediator, rather than giving them control over the natural wealth they are due.
It is not even all that clear that social democracy is stable. It seems especially sensitive to rapid demographic changes (aging population, too low immigration, too high immigration, etc.)
Rather than put band-aids on the fundamental inequalities in society, we should focus on the root of these inequalities in the first place. The class monopolies that states produce through the granting of privilege via its monopoly on the legitimization of violence are where we should look first. Why exactly does the capitalist class have such disproportionate bargaining power which allows it to exploit the working classes? Because the state used violence to give it these monopolies on resources, intellectual property, the movement of labor, and social institutions relating to finance. What happens when the state no longer gives them these monopolies? What happens when the capitalists must join the workers because their position and privileges have been degraded? That's a more interesting question!
In my opinion capitalism is in some way necessary because people don't want take risks if there is no reward. So if everyone would own everything society wouldn't progress because no one would pay more taxes than others because no one would start up new companies and earn more money. That's why social democratism is the best possible social system.