In fact, economists do exactly that with their models, but they express their predictions in probabilistic terms. They point to the many margins of error that exist around all the variables in their model, and all the assumptions they must make, and admit that their ultimate calculation contains all of these sources of error by construction.
To be effective, it needs to take into account the fact that Greek firms have both significant non-performing loans vis-à-vis banks and significant arrears to the state.