On the one hand, Quantum Field Theory is just a special case of quantum mechanics. It’s just the quantum mechanics of fields (corresponding to situations whose classical analogues involve an infinite number of degrees of freedom). So replacing quantum mechanics with QFT is like replacing dogs with dobermans. Yes, we could replace all other dogs with dobermans, but they’d still be dogs (and for some purposes less useful than the ones they replaced). On the other hand, in a situation with only a limited number of degrees of freedom (such as where there are only low energy interactions between a fixed number of particles – in the analysis of a chemical bond formation for example), the use of quantum field theory would be like keeping track of the motions of all the engine components in a car when all we are interested in is the effect of a collision on a crash test dummy (or replacing a dachshund with a doberman for flushing rabbits out of their burrows).