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.
Who mentioned that everybody would own everything under socialism? Socialism merely is the premise that the workers in associated labor should control the productive capital which they use to perform that labor. People could and would still take planned risks because there are still rewards achieved by reducing the costs of production. It just so happens that you can only profit under socialism by actually reducing costs rather than externalizing them on to others.
There are also many ways people can be motivated to achieve something without financial profit.