"Human thriving" can be defined any number of ways, including wealth, health, liberty (positive or negative), freedom, happiness, environmental factors, etc. Most capitalists define thriving as improvement in some or all of these areas. You are correct that a vague definition of "thriving" can be self-serving, but it is hard to find any factor which socialist countries have made more progress in than less socialist countries. Even a socialist country (i.e. China or India) which frees its markets and creates property rights sees a substantial gain in the rate of improvement of the lives of its citizens in almost every aspect.