What prevents the substitution of a PNP transistor with an NPN transistor?

Bipolar Junction Transistors (BJTs) come in two varieties: PNP and NPN. The key difference between them is the arrangement of their P-type and N-type semiconductor materials, which fundamentally dictates how they operate in a circuit. Because of this structural difference, an NPN transistor requires a positive voltage at its collector relative to its emitter, while a PNP transistor requires a negative voltage. Substituting one for the other without changing the power supply connections would mean the applied voltages are completely backwards. Therefore, you cannot simply swap an NPN for a PNP (or vice versa) because the required bias voltage polarities are reversed. Distractors like current gain or frequency response do not represent the hard physical barrier that reversed polarity creates.