As mentioned before, this could put the analyst in a situation where true coefficients, i.e. considering the DR effect, are not estimated correctly. Sticking to the linear coefficients will result in investing the entire budget to the channel presenting the highest coefficient and not allocating any money to the rest.
Note that some analysts tweak curves post-modeling and manually add the DR component. This is very dangerous as saturation in this case is not derived from data but based on the subjective judgment of the analyst.
From a realistic business perspective, this is unacceptable. Hence, while running their models, the analyst needs to create media variables presenting DR and regularly check the results of the Optimization following an agile approach.