The same reasoning might apply to modular structures in biology. It is perhaps easiest to think of an ecosystem first, where each species is like a company. If one species provides a function that is needed by the other species, then the ecosystem depends on that first species. In other words, the ecosystem will probably evolve so that there is a reserved seat for the first species. Now, moving the analogy to population of cells or population of genes within cells is a bit different, but I think some of the analogies still apply. The underlying idea is that the system favors those species that are necessary for the whole system to survive. And the reduction of cost is the incentive for new species to evolve and take a specific role in the system.