While it is true that classical mechanics emerges as a limiting case of quantum mechanics when certain conditions are met, the claim that there are no purely classical systems in the real world is based on the recognition that all physical systems are fundamentally governed by quantum mechanics.
In the macroscopic world we observe and interact with on a daily basis, classical mechanics provides an accurate description for many phenomena. Classical mechanics is a highly useful and successful theory for understanding the motion of everyday objects, such as cars, planets, or baseballs, where quantum effects are negligible and can be effectively ignored.
However, it is important to recognize that classical mechanics is an approximation to the underlying quantum mechanical reality. As systems become smaller, lighter, or interact with their environment differently, quantum effects become more pronounced and cannot be ignored. At the atomic and subatomic scales, the behavior of particles and systems is inherently quantum mechanical.
The claim that there are no purely classical systems in the real world reflects the understanding that even macroscopic objects, while well-described by classical mechanics, ultimately arise from the behavior of their constituent quantum particles. The interactions and entanglements at the quantum level give rise to emergent classical behavior when averaged over large numbers of particles.
In practice, it is extremely challenging to isolate a system entirely from its environment and achieve a truly classical behavior. Quantum effects can manifest even in seemingly classical systems under certain conditions. For example, superconductivity, where electrical resistance vanishes, is a macroscopic phenomenon rooted in the quantum behavior of electrons.
So, while classical mechanics is a powerful and effective tool for describing many phenomena, the claim that there are no purely classical systems in the real world stems from the recognition that all systems, regardless of size, ultimately obey the laws of quantum mechanics.