This decline is a necessary aspect of Capitalism. Competition, for all it helps initially with forcing prices lower, ultimately comes at the cost of increased exploitation of the Working Class. As all value comes from labor, there is only so much you can immediately automate away to lower cost of production before you must further exploit your labor force to remain competitive.

Socialist markets, ie ones controlled by Worker Co-operatives, still face these issues, but delay them due to being of and for the workers themselves.

Only a non-market form of Socialism, such as Anarchism or Communism, can actually permanently solve these issues. Markets are a useful tool during Capitalism, but just as feudalism gave way to Capitalism, so too will Capitalism give way to a more equitable distribution of control.