I was discussing general design principles, and my 4E experience is limited to three sessions (because of DM issues), so I don't have much to say about 4E. About 5E: one implication of the flat curve you mention is that 5E is also friendly to parties with heterogenous levels. You could have a 20th level Gandalf, a 14th level Aragorn, an 11th level Boromir, and a couple of 1st through 3rd level Hobbits in the party, and they'd all be able to contribute meaningfully. If proficiency bonuses were massive (+1 per level) then Gandalf would dominate the whole party in every field of endeavor from social intercourse to engineering to mountain climbing. Systems with a "god stat" that allow super-generalists like this tend not to be very fun in my opinion, unless you are explicitly going for over-the-top James Bond-ism (GURPS: Black Ops).
I think your example is...more than a little far-fetched. Even if we make all of the characters involved "normal" characters (that is, ones which follow the rules of character creation even if it means not doing everything Gandalf etc. could do), the level-1 Hobbits are going to have net bonuses to their
best stuff (max stat+proficiency) of roughly +6, while Gandalf and Aragorn rock roughly +11. If he's an Evoker (reasonable, he makes fireworks) Gandalf can do more damage with a low-rolling
cantrip than those Hobbits can do on a
crit. And that has nothing on the Hobbits being at risk of immediate death if fighting anything above level 6 or so.
In certain narrowly specific ways, yes, this party "works" more than it would have. Most, but not all, foes can be hit by the hobbits with non-crit attacks, but many of them will still be in the 10-15% range. Most, but not all, enemies will not be
guaranteed to hit the hobbits, but their odds of success will be in the 70%-or-higher range. Skill checks that Aragorn finds challenging (say, 40% odds of success) will be nearly impossible for the hobbits (roughly 10-20%
with proficiency and a +3 stat; impossible, or nearly so, if lacking both). So...yeah, I don't think your example is especially great.
I can accept, however, that characters within ~5 levels of each other, as long as none of them are below level 3, are close to equivalent in strength; the HD make the biggest difference, and the higher level you are, the smaller that relative difference becomes. The first three levels have such tiny HP values and such enormous risk of death (a typical orc can drop many 1st-level characters with a single crit; two hard hits can outright kill a fragile class) that it is...unwise, IMO, to mix "higher than level 4" characters with characters at level 1 or 2.
TL;DR: No, I really
don't think Level 20 Gandalf (or even level 14 Aragorn) can adventure confidently alongside level 1 Samwise and level 2 Frodo. But I do think level 6 Subotai, level 7 Conan, and level 4 Valeria can adventure together more or less alright.
Anyway, that's why I think it's an important feature of 5E that they consciously made the variance so high, and the d20 roll such an important component of any skill challenge. And from there it follows that even a Shadow Monk with a total bonus of -1 to Intelligence (Nature) can still look for poisonous herbs for his herbalism kit in the wilderness with some hope of success, if he thinks to do so. It may or may not be realistic, but it's definitely a distinctive feature of 5E play.
Yeah, again, this depends on the difficulty of the check. With a total bonus of -1, if you're trying to do something Hard (DC 20) you cannot possibly succeed. I wouldn't be at all surprised if a DM said that trying to scrounge up poisonous herbs in a forest was a Hard task. Even if it were only Medium (DC 15), you'd still only have a 20% chance of success. So there's still skill things you cannot attempt, and the numbers aren't far off for things you can attempt with slim chance of success (compare to 4e: for a 10th-level challenge, a Hard task is DC 26, Medium is DC 18; even for -1 from stat, a 10th-level character has +5 from half-level for a total of +4, so while you cannot succeed at the Hard task, you have a 30% chance with the Medium task. These diverge a bit more as you advance, but at the same time, nobody has negative modifiers after level 21, and
everything gets the half-level bonus.)