This claim is simply wrong. Have you read the Baran and Sweezy book? If you had, you'd understand how wrong your claim is. Do you have any basic idea of what Keynes argued, for that matter?
If the rich are too rich, they have trouble finding new ways to make new traditional (goods and/or services-producing) capitalist investments. The existence of a supply of capital, as Keynes argued, does not in itself create an investment outlet. The existence of the investment outlet requires the prior existence of unment effective demand.
There's more to the argument than that (surplus isn't equal to final after-tax property incomes), but the over-accumulation argument is Keynesian, as is reality.