Marxism isn't when you have coops under the capitalist mode of production, because things are still being produced for the sake of profit, not for social and common, and it still has the effect of making living labour serve accumulated labour, among other factors.
That being said, worker coops are a good thing and are often more stable and efficient than conventional businesses^1 (not to mention leading to better outcomes for the workers). The problem comes with economy of scale when it comes to competing against accumulated labour (startup capital is a problem, and by the nature of taking startup capital you are working to multiply the exchange value of accumulated labour). The living labour of the worker coop will still likely be at work to maximize the exchange value of some other's accumulated labour (loans, contracts, etc) or become of the M-C-M model themselves, siding with accumulated labour class (bourgeoisie). That's capital for you.
That being said, worker coops are a good thing and are often more stable and efficient than conventional businesses^1 (not to mention leading to better outcomes for the workers). The problem comes with economy of scale when it comes to competing against accumulated labour (startup capital is a problem, and by the nature of taking startup capital you are working to multiply the exchange value of accumulated labour). The living labour of the worker coop will still likely be at work to maximize the exchange value of some other's accumulated labour (loans, contracts, etc) or become of the M-C-M model themselves, siding with accumulated labour class (bourgeoisie). That's capital for you.
1: https://www.democracyatwork.info/what_do_we_really_know_abou...