Agreed, that's too flat.
That said, and this is where lack of granularity rears its ugly head, I prefer there always be a tiny chance of success or failure no matter what. Here, that rube should have a 1% or even just 0.1% chance of bluffing Asmodeus - but the system as written doesn't allow that, it's either 5% or zero. Flip side, no matter how good you are at something there's no such thing as perfection, reflected as a chance of mechanical failure again smaller than the non-granular d20 will allow.
Bell curves are better when they have long tails.
Completely agree. It's the same issue as 3e had with the d20 - trying way too hard to shoehorn everything into a unified mechanic rather than using bespoke mechanics in situations where they just work better.
This I don't mind so much. The opposite is 3e, where a given creature would only be a viable opponent for a window of about 2 character levels, before which the PCs wouldn't have a chance against it and after which the PCs wouldn't even work up a sweat.
I'd rather that "viable opponent" window be considerably wider, which requires some combination of a) flattening the power curve and b) making combats swingier so as to increase the chance of upsets (either way).
I'm sure you would rather that.
I'm even more sure that it isn't gonna happen. Folks are already souring on what 5e's designers sold them on this front. "No no, if we just do it
even more, then people will LOVE it!" is not and has never been a successful policy argument.
Also, you mischaracterize 3e. I have little love for 3e as a mechanical system, so I assure you that a window of only two levels is simply not accurate.
The only reason you'd have windows like that is because of the horrible, horrible, horrible
spellcasting stuff that would make a creature only really viable as long as you had access to the right spell level. In terms of AC and such, the range is far broader, in part because of the design behind 3e's (very stupid) "iterative attacks" thing.
Certainly, I can say 4e was verifiably much better than that. The 4e rules explicitly instruct the GM to vary typical combats in at least a 5 level range (roughly party level ±2), and further, that the GM should include combats that are as much as 8 levels above or below the party, because routing your enemies every now and then feels really good, and getting routed every now and then reminds the party that they aren't invincible. When added to the natural variation that arises from luck, you get just the right
feeling of the world being challenging but worth tackling, as opposed to forbidding or forgettable.