Too much math for 5e's design philosophy, and with bounded accuracy it would need a crit on the strongest enemies anyway.

The actual implementation of that system could take a number of forms:

• PB/LR for every class that is proficient with the given weapon. (The most basic implementation, but also screws over pure martials in favor of half casters.)
• PB/LR for every class that is proficient, but fighters regain PB/2 uses when they use Second Wind.
• PB/LR for every class that is proficient, PB/SR for fighters, monks, and barbarians.
• PB/LR for every class that is proficient, fighters regain all uses when rolling initiative.
• Etc...