If you have big data about all transactions occurring in real-time, along with socioeconomic factors on all the actors, it is not difficult to verify models and see how well the relationship between their dependent and independent variables holds up in the real world.
That is a big if. Nobody has data for all transactions occurring in real time and nobody knows all the socioeconomic factors for all the actors.
Even if you did have all of that data, you would still be missing a lot of data. For example, an important factor in economics is asymmetric information. An actor's decision may be optimal given the information available to them at the time even if the decision is suboptimal given perfect information. So for an even more accurate model, you would also need to have data on what each actor knows at any given point in time.
Furthermore, an actor probably does not have the ability to calculate the optimal outcome given the information available and there are innumerable subjective factors in economic decision making.
Big data will help you find some kind estimate, but it will not suddenly make economic modelling simple.
