[kergonath]

What you say is true, but also incomplete. We are perfectly able to quantify the accessible states in purely classical systems, such as ideal gases, without requiring discrete energy levels. The trick is to think of a continuous probability density instead of discrete probabilities. This framework is very general and does not depend on the quantum-ness of what you look at.

Even in some systems that actually follow quantum mechanics (such as phonons or electrons in a material, or photons in a black body), we often use continuous probabilities (densities of states) because it’s much more convenient when you have lots of particles.