Agreed. I don't want to be making judgement calls on whether a PC's skill is "good enough" to skip making a check. The whole point of a skill system is to take care of that for me so I can concentrate on running the rest of the game. If a PC is so good that they can't fail, then the skill system should reflect that by making the mod high enough that they can't fail. If it doesn't do so, that's a problem with the skill system.Sidestepping the knowledge argument for the moment, I think there are a lot of ideas and house rules here that go against the whole point of the 5e skill system.
In 5e, DCs are fixed; they do not scale up. That's the whole point of bounded accuracy. The DM should absolutely never consider the character level, attribute score, or proficiency of the character when setting the DC. The only question the DM should ask himself is, "Is this task easy, medium, hard, or even harder?"
The way to understand 5E skills is that the rogue (and to a lesser extent the bard) is the party skill-monkey. The fighter is a specialist in fighting; the wizard is a specialist in spellcasting; that's their thing. Rogues are specialists in skill usage, and they are given the tools to shine at it. That's why they get Expertise and eventually Reliable Talent. Everyone else is a dabbler. If you start giving other PCs a pass on "easy" skill checks (whatever that means), you're basically handing out a version of Reliable Talent to everybody.
The number of scenarios where this leads to wonky results is very small. Who here has seen a single in-game arm-wrestling match within the last year? In the rare event that it does happen, I'd just rule that it isn't a check at all and use a straight-up comparison of Strength mods. If your Strength mods are equal, flip a coin. You can add your proficiency bonus if you can justify being proficient in arm-wrestling.
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