Answers:
A replacement model is based on the probabilistic model meaning that everything is left to chance. For example, when dealing with parts that breakdown suddenly, we invariably use the probability that may or may not happen at all.
Replacement models do not take into account human factors. For example, there are items or machines to which emotions are attached and everything cannot be treated in terms of costs and profits. Replacement models fail on this aspect too.
Replacement models deal with large scale calculations, which can only be done with the help of computers and large calculating machines. This too involves many costs.