M-class destroyers as a point of design maturity

Is it accurate to call the M class the beginning of the Royal Navy's torpedo-boat replacement program?

Not exactly; earlier classes were already moving that way, but the M class marks a commonly recognized point at which the torpedo-boat destroyer had matured.

The M class did not appear without predecessors, and several earlier designs already pointed toward the mature fleet destroyer. Its importance lies in crystallizing those changes into a recognizable standard. The class is therefore a convenient marker of maturity rather than an absolute starting point.