On my phone so short shrift: how does your Aragorn afford straight 18s in Cha, Dex,AND Int? Given his MADness I thought assuming 16s and 18s was pretty generous, but you seem to consider that insufficient.
You should review the MM ACs, it will change your ideas about the uselessness of a couple of hobbits with missile weapons.
He's a human (though could be sold as a half-elf, what with being descended from Elros). If we presume the default array, he started at level 1 with 16, 15, 14, 13, 11, 9. (This would be easier if he'd rolled lucky, or used point buy, but we'll stick with it for now). Presuming he put his best scores as Dex, Cha, Int, he could have 18 Dex at level 4, and 18 Cha by level 12 (and be partway to 18 Int, too). I figured he wasn't really pushing Int though, and focusing only on Dex/Cha. If he had rolled even moderately lucky (that is, done more than slightly better than the array, which is the "expected value" for rolling), he could potentially have all three 18s, or one 20 and one 18, since he'd have a total of +6 stat points to distribute. (For example, if he rolled 16, 15 as his highest two stats, plus the human bonus for 17, 16, he'd have more than enough points to be 20, 18 at level 14.) Meanwhile, Gandalf--even with the array--easily has 20 Int and 20 of whatever one other stat he wants, since everyone gets a total of +10 stat points to distribute (except Fighters, who get 14; if Aragorn is built as a Fighter instead, he could absolutely have two 20s by level 14, regardless of method used, and possibly a 16 in a third stat if he rolled well.)
There's pretty much no way I'll be able to actually check an MM, so I'll have to take your word for it. When the free rules include three threats, of challenge levels not-inappropriate for first- or second-level characters to face, that a first-level character can't possibly hit (on average) more than 40% of the time if they rolled lucky (or picked highly synergistic race/class options), I'm not particularly impressed with the claim that a sub-level-3 character can adventure alongside a higher-than-12 character and "contribute meaningfully."
I haven't found low level members of heterogenous parties to be as fragile as you think they are. Could be a playstyle difference but let me all you: how often have you seen the low-level guys die? Not just get knocked out, but actually die? I haven't seen one death yet, much less permanent death. I don't have a twenty-level disparity but my guys do fight CR 8-10 creatures and other deadly threats, which is only slightly weaker than what the hypothetical Gandalf-led party would be fighting by DMG guidelines, so I think my experience is relevant. It's quite hard to kill PCs in 5E unless you deliberately try--e.g. hitting hobbits when they're down--and why would anyone do that unless you're fighting them on death ground? (And good guys don't usually fight people on death ground at all--they accept surrender instead.)
Sadly, I haven't played tabletop anything for a good three months, and no D&D for over a year--lack of group, moving, schedule conflicts, etc. So I haven't actually seen either way. What I can do, though, is look at to-hit values and damage values, and consider the reports I've heard. Admittedly, people with "extreme" situations are more likely to mention something, but I've heard a
lot of complaints about TPKs in the Hoard of the Dragon Queen adventure because of the...Redbrand Thugs, I think? And the fight you're apparently supposed to run away from (a half-dragon, IIRC?) where that is poorly telegraphed.
As for "quite hard" to kill PCs without trying--when they're at least level 3, sure, it's not
trivially easy to kill people anymore. But for a level 1 or level 2 character, even with a +2 Con and d8 hit dice (since the Hobbits are pretty clearly not Fighters, Rangers, or Paladins), they only have 10 HP at level 1 and ~17 at level 2. A couple of hobgoblins, which I'd call decent approximation of Uruk-hai, can hit for 1d8+2d6+1, on average 4.5+7+1 = 12.5. A lucky crit (8+12+1 = 21) kills a first-level character outright, no death saves at all (-11 HP is greater than the total), and drops a fresh second-level character to Dying. Two below-average hits puts a second-level character at Dying; a slightly above-average hit on a Dying second-level character is also instant death (17+ damage happens ~12% of the time, before counting crits).
The hobgoblin is a CR 1/2 creature. Two of them are worth 200 XP; since there are two, the difficulty multiplier is x2. So the "fight difficulty" is 400 XP. This is exactly the threshold for a "medium" encounter for four level 2 characters, and exactly the
deadly threshold for four level 1 characters. I think it is thus fair to say that facing off against a mere two Hobgoblins/"Uruk-hai" is "deadly" for the 1st-level people, and very dangerous for the 2nd-level people, while being a breeze for a 20th-level person and not particularly challenging for the 14th-level person.
If the hobbits were at least level 3, which is the "this is where things change" point I've been using this whole time, then a pair of hobgoblins ceases to be super duper dangerous. I can pretty easily buy the idea that many challenges are also much less scary at that point--not non-lethal, to be sure, but not a meatgrinder either. They'd also have all their specialization stuff squared away by that point, so they'll have all the basic tools 5e gives them (more or less) for interacting with the world, whether combatively or skillfully.
Again: I am
not trying to say that 5e can't accomodate a spread of character levels. It absolutely can. I just think your "some level 1 and 2 alongside level 20" example is hyperbolic to the point of not actually supporting your argument. If, instead, it had been level 3-4 hobbits alongside a level 7 Boromir, level 10 Aragorn, and level 14 Gandalf? I could buy it. It'd still be too wonky *for my tastes,* but the group could make it work, without "coddling" the hobbits and without throwing them headfirst into lethal danger.
[sblock=Digression about 4e]I am, however, also saying that
if you adjust the level range for the altered numerical scaling, 4e and 5e don't actually accommodate THAT much of a different range. In 5e, it is "best" (in the sense of "everyone is damn close to the same footing") to have a range of +/- 2 (range of 4) levels, or as I said before, "everyone within 4 levels of each other." The first two levels are excluded from this, because they are so squishy and have fewer tools (sometimes far fewer) regardless of what "pillar" you look at. If you're comfortable with some people lagging behind noticeably, but not dramatically, then you can double that range to +/- 4 (range of 8; again, excluding levels 1 and 2). Since 5e scales about half as fast as 4e, its ranges should be about twice as big--and lo and behold, 4e is pretty much identical for characters that are all within +/- 1 (range of 2) level of their average, and can work (though will have some noticeable-but-not-dramatic hiccups) for +/- 2 levels. And as I've noted with DCs, the half-level bonus in 4e means tasks that had previously been impossible become quite achieveable. (If you look at "tasks that are very hard
for a particular level," then non-specialists do fall behind, but the world is not typically scaled to fit the character, so that's not as important to me IMO.)[/sblock]
Great point about the delayed death rules in 5th Edition! They let low level characters contribute a lot more, because they don't have to be so afraid to get into the action.
Believe it or not? This is one of the
very few mechanical structures taken precisely as-is from 4e. Death saves are how dying works there. (Minor caveat: I don't *think* 4e had "massive damage = instant death" rules, but it might have, possibly as a later option.)