The go-to tool in the economic modelers toolbox is to simply ignore terms that are difficult to model and/or calibrate or which are politically inconvenient, such as all that pesky 'investment' behavior. Once investment at risk has been made one time, a Marxist sees no need to ever consider it again'; it is only time to consider carving up the carcass, or distributing the mal-distributed fish that are nicely packed in ice on the docks, far from the storms at sea.
The proof of this is the darling of modern economic modelers: "the endowment economy." A pure consumption model, where not only investment but even savings is 'impossible.' Also known as, a lost episode of Star Trek.
In an 'endowment model' there is not only no need to model investment decisions or behaviour...there is no investment decisions or behaviour, othet than, Trust Fund babies either consuming or lending each other money. Period.
This is not a 'fringe' economic model. It is the darling of Marxist economists.
It is the basis for Krugman's "Debt De-leveraging Liquidity Trap" paper, and his "Paradox of Toil... Paradox of Savings" complete nonsense. It is how he tours the Hustings and beats folks over the head with the assertion that in our current 'The Economy' any lowering of business taxes will 'constrict The Economy.'
The Economy he is modeling is an 'endowment economy.'