Textbooks frequently argue that QFT is necessary because it's the only way of combining the quantum mechanics of particles and special relativity. This is true, but there's a much simpler argument for QFT's necessity: Quantum field theory is necessary if you want a correct and logically coherent description of the physics of fields.
In particular: If an electron is in a superposition of position states, then its electric field must also be in a superposition. So we need operators to represent value of the electric field at various points. There are technical subtleties here, but the core point is very simple: As soon as you need to discuss the possibility that a field is in a superposition of states, you are doing quantum field theory. You can obscure this by choosing bad language, but the mathematics doesn't care.