Language has had its own evolution. The most significant adaptive advantage of language is for language itself. This partly explains the regularities and complexities of languages, since all languages have had several thousands of years to crystallise. The population of speakers is thus seen as the selecting environment of language which is an implicit part of how the computer simulations are designed. A convenient metaphor is to view language as a virus dependent on a population that reproduces it.
Language is a cultural virus, analogous to biological ones.
Differences in languages appear because some groups are more or less isolated, and therefore have preserved slightly different variations. Given these conditions for the simulations it is possible to explore why German case marking did not succeed Scandinavian case marking. By introducing a separate marker for the distinctness of the German system, and transfering that in parallel, we can have an estimation of how those markings travel. One factor is the larger Scandinavian population. Germans would become more integrated into the Scandinavian language community.