Adding a whistle (or whistles)is going to suffer from the fact it was trialed in the past (2 whistles plus 2 linesman—I’m pretty sure the trials were before we moved to “AR”) and were considered an abysmal failure such that I believe the trial was stopped early.
That said, the trial was in the era when ITOOTR was a core philosophy of the Game. (For those with less gray hair, the definition of fouls used to include “if in the opinion of the referee.”) As the powers that be have tried more and more to turn the subjective into the objective, there may well be less variation in foul thresholds when the trials took place, and the existence of voice activated comms can matter too—each ref would know exactly what warnings had been given, etc. So a trial might look very different today. In the U.S. today, I’m pretty sure there are still high schools and colleges (not around me) that use a three whistles system with, roughly, the ARs coming onto the field with whistles—in some places they rotate rolls a third of the way through the game. And there are many places in the U.S. where high school and college are done with the dual system (two Rs, no ARs). Done improperly, each R stops at or about the halfway line; done properly, the lead R advances in position akin to an AR and the trail R has a position akin to a single R—much better coverage, but brutal in transition, which is why many refs are lazy and stop at the halfway line. (Many rec leagues, especially ones not affiliated with USSF use the dual system as well; affiliated leagues are prohibited from doing so, but some do anyway.) Most refs I’ve talked to who have tried these (or worked extensively in them)much prefer the R/2AR model. My experience with the dual system (rec and high school) is that it isn’t terrible at that level so long as the 2 Rs are on the same page. But when they aren’t (different foil thresholds, different views on what “deliberate means on HB), different dissent thresholds), one wants to run the dual properly and the other doesn’t, it’s a hot mess.