This post claims that there is no real photon (particle) with a plane wave solution well-defined momentum state).
It makes sense somehow to me. I can think of several arguments:
The plane wave solution doesn't have well-defined probability as it exists in infinite space with equal probability. We will never be able to detect those particles.
Our probability interpretation breaks down with the plane wave solution. It's not square-integrable.
Even though it seems non-sense, we apply quantization rule to the free particle which makes quantization as the imaginary process which doesn't exist in reality.
Besides 'it works' argument, do we have any explanation why this quantization process makes sense?