Pedantic
Legend
That's the tension between two gameplay modes in action. If there's a negotiation based mode available, and it's shown to routinely produce better outcomes or is just more understandable to the player, then it becomes easier and more normalized to use it as the default means of expression/gameplay. This fighter example involves mechanics the GM doesn't get to say anything about. All they could do to alter the resolution is clarify the situation is different than the player understood it to be (give or take 5e's persistent fuzziness about basic resolution)... Which provides more context for any other use of mechanics that player might have in mind.Interestingly, a 5e fighter can do much of that. If they use their action surge to dash and get extra movement, they may have enough movement to cover that 20 ft climb and leap onto the monster's back and still have the action to stab.
You know, it could be an interesting thing to tie invoking negotiation to a currency and/or a GM side flag (though you'd probably want a bigger unit than action by action). Sort of takes it back to early action/hero point currencies, but I haven't seen anything that explicitly lays out conditions on when you can/can't try to stunt. On the GM side, I'd probably want to use something like Fantasy Craft's dramatic scenes, which activate a whole bunch of mechanics when declared for an extended duration.