Look, there is a reason why the whole Punisher-fandom loved the heck out of the netflix show.
Not because of the perfect casting, not because of the multiple-layered character, not because of the decent writing, and not because of the r-rate.
Because of ALL of these. This, his VIOLANCE and brutality what makes Punisher DIFFERENT than any other marvel-character who uses guns. This is why its technically TABOO to work with him. Remember that line in original Civil war? "Tell me we aren't so far gone that we signing up The Punisher?"
Besides, in a medium that so much about visuals, the visual effects that r-rate allows is which able to show that difference, that why he is different.
Silver Sable uses guns too. The Winter Soldier too. Black Widow as well. The MCU's Falcon also have guns. neither of them need r-rate. because neither of them like The Punisher.
.
Sure. when or IF Punisher appears in an Avengers movie, he have to fit into THAT movie and THOSE characters' ratings. But his OWN content still have to keep the r-rate. Because this is how he ALREADY worked perfectly in his own netflix-show, and by having his own show it's also not needed to CHANGE it and downgrade it.