Maxwell's equations did not answer all electromagnetic theory. Actually Maxwell equation + Lorenz gauge + superposition principle together solve the current electromagnetic problems. Hence superposition principle is an independent principle. It is not included in Maxwell's theory.
Lorenz gauge is very important. Lorenz gauge is a correct gauge. Maxwell uses the Coulomb gauge. That is a wrong gauge. Hence in order to solve the electromagnetic problem, Lorenz gauge is required. Lorenz gauge should be as 5th Maxwell equations. (Here is Lorenz not Lorentz). Lorenz introduced Lorenz gauge in 1867. Only a few years later than Maxwell's theory. He also introduced retarded potential in the same paper. The concept of retarded potential is equivalent to the displacement current of Maxwell. Lorenz introduce the retarded potential is base on his study in optic and elastic wave. Hence Lorenz should get the respect similar to Maxwell.
As I know the suppositions of the static field electric field and static electric magnetic field is self-explained. However it is difficult to prove the radiation field can be superposed.
For radiation fields, there are two kind of fields retarded potential and advanced potential. They are the solutions of Maxwell equations. As I know the transmitting antenna sent retarded wave. The receiving antenna sends the advanced wave. Most scientists do not accept the advanced wave because of the causality problem. However the absorber theory of Wheeler and Feynman, Transactional interpretation of John Cramer supports the advanced wave. The mutual energy flow theorem also supports the advanced wave. We know there is current in the receiving antenna when it receives wireless waves. This current should produce some wave, if this wave is retarded wave, the receiving antenna actually is a transmitting antenna but not a receiving antenna. Hence the advanced wave should exist. Can the retarded wave and advanced wave be supposed? This is not clear.
Radiation is a very complicated phenomena, our today's theory still does not answer the problem of wave-particle duality problem. Wave can be supposed, but it is clear that the particle cannot be supposed.
Feynman and his doctor father Wheeler tell us if the space is empty, the sun cannot shine. In order to let the sun shine there needs the absorbers. Our current electromagnetic field theory (Maxwell theory) did not reflect this point of view. This means something is wrong in classical electromagnetic field theory. If something is wrong we first need to test whether or not the superposition principle is correct. There are some publications that tell us the supposition principle is correct at 15%, (I forget the cite place). Anyway superposition principle is not tested like Coulomb's law.
There is the concept of wave collapse. The wave collapse process if it exists should be some kind of waves. As I know the wave collapse is related to the concept of time-reversal waves. Can this kind wave be supposed? It is not clear.
Summary: Maxwell theory is not a complete theory (Otherwise do not need QET), superposition principle should be an independent principle to Maxwell equations. It needs to be tested again and again in case there is some new theory developed in electromagnetic field theory or quantum mechanics.