I'm struggling to understand the DM mindset that would be so arbitrary and capricious as to design an unwinnable encounter (for story reasons!) but refuse to arbitrarily or capriciously assign creatures in said encounter immunity to perfectly mundane character abilities that would make the encounter winnable. The sort of person who would "let the dice fall where they may" would never design such an encounter in the first place, and if they would they'd do so with a full understanding that they might have to kiss their BBEG good-bye. Because that's kind of how "recurring" villains work in RPGs; you don't put them in PCs' path without the expectation that they will find some to kill them.
So sorry, but I don't really buy this as a legitimate concern.
The real problem here is the fact that people tend to throw away the concept of "rulings over rules" as soon as actual rules exist. @
Tony_Vargas has called 5e at various times the "DM Empowerment Edition" which either is or is close to the truest thing anybody has ever about 5e. There are arguments to be had about whether that's a bug or a feature but whether it exists or not is not in question. I happen to think of it as a feature myself.
The thing about "rulings over rules" is that people only tend to bring it up when 5e doesn't tell you how to do something. But it's not "rulings over the lack of rules"; it's "rulings
over rules". The DM is [empowered to be] responsible for supporting and maintaining the internal consistency of their world. There is literally nothing stopping a DM from assigning disadvantage to Smuggy McTerrorpants's Menacing role and Dark Evilplans advantage on the opposing role, regardless of the printed stats. Or say that no roll is necessary, because it's not going to work.
Emphasis added. The implication is quite clear; when the outcome is
certain (that is, the puny low level bard is ever going to actually terrify the mighty, nigh-immortal warlord), the dice
do not determine the results; the DM does.
Rulings over rules.
This might seem arbitrary in a white room where a feat seems to give a PC an
ability, but in actual play it's the only ruling that makes sense given the internal consistency of a world, which is something I think everyone here would agree on, and absolutely nobody with any sense in their head would fault you for the ruling.
This whole argument reminds me of another nonsensical anecdote from 4e days. See, in 4e days the rules explicitly stated that the only way to end ongoing fire damage (say, from being lit on fire) was to make a successful save. What happens when a character taking ongoing fire damage (from being lit on fire) decides to jump in a lake? Do you follow the RAW, absolutely? I suppose the answer depends on another question: what sort of game are you playing?