on an individual level maybe it's (partially) subjective, but marx is concerned with the systemic level. he's talking about the industries which really drive the economy

i don't think there's ever been a successful attempt to reduce macroeconomics down to the microeconomics level. basically, the system as a whole cannot be calculated by summing up a model of all the individual behaviors, it doesnt work that way. there have been several broadly successful macroeconomic models

steve keen talks a lot about this same type of stuff and he's very much not a marxist, if you want a different perspective on it