Even without doing this though, I don't think 18s are really all that needed, desired sure, but not needed. With the increase in thac0 that warriors get they end up being quite capable at striking their enemies and once you start getting magical weapons or adding in specialisation bonuses I think they quickly start being able to take care of themselves. While I do think that stat bonuses could have been gained a point or two lower, I've never found them necessary to have fun playing the game.
That's an interesting perspective, and I think there's some truth to the idea that AD&D is playable with characters having mediocre stats, but it's certainly playing on hard mode.
Celebrim has compared and contrasted the massive difference in DPR between a Fighter with no Strength bonus vs. one with Exceptional Strength, and the encumbrance difference. As another contrast, say look at a Cleric with a Wisdom of 14 starting out with three 1st level spells every day, vs one with 9-12 Wisdom having not just no bonus spells, but a flat percentage chance of failure (between 5% and 20% depending on how low that Wisdom is) every time they cast a spell. A Magic User with a 9 Int can only ever learn spells up to 4th level, and has only a 35% chance of learning any given spell they try to add to their spell book. At a 13 Int they're capped at 6th level spells and still have only a 55% chance to learn a given spell. This is definitely not a system intended to be played with Prime Requisites under 15, and as Celebrim also pointed out, the large majority of ability score benefits are gated behind scores of 16 or better.
If I was to run 2e again, I'd simply remove exceptional strength from being coupled with an 18 and allow warriors to roll it no matter their strength score. Half the time, exceptional strength only gives a +1 damage bonus anyway so I don't see this as a big deal and you can play around with it a little so that single classed fighters gain a bonus to the percentile roll to give them a small bonus.
This is a fun solution. Reminds me a little of Grit from Shadowdark. Just give Fighters and their subclasses (the Warrior class group in 2E) a flat benefit to Strength all the time. Not as elegant as just "advantage on checks", but functionally assuming all Warriors have 18/xx Strength and giving Fighters (or single-classed Fighters) a bonus on their percentile roll would work.
As an aside, don't 1e rangers get to add their level to damage against giants which ended up being almost any monstrous humanoid in 1e? I'd have thought that they'd quickly start dealing a decent amount of damage fairly quickly after just a few levels, though admittedly less useful against things like undead, they'd excel in their element.
1E Rangers get a +1/level damage bonus against "humanoid-type creatures of the 'giant class'", and it goes on to list them. Bugbears, Ettins, Giants, Gnolls, Goblins, Hobgoblins, Kobolds, Ogres, Ogre Magi, Orcs, and Trolls. This list got expanded a little when the Fiend Folio came out.
AD&D didn't explain what the heck "Giant class" meant, so it meaning all those humanoids and not just giants made no sense to me until many years later when I read OD&D and Joe Fischer's original Ranger class from The Strategic Review, and learned that it originally referred to the wilderness encounter type tables, specifically the one labeled "Giants", and the ability was originally described as applying to all creatures of the "Giant class (Kobold to Giant)", which meant everything on that specific wilderness encounter sub-table between and inclusive of those two entries. That is, Kobolds, Goblins, Orcs, Hobgoblins, Gnolls, Ogres, Trolls, and all Giants. (and excluding the non-evil entries at the end of that table - Gnomes, Dwarves, Elves, and Treants.)
While the table in the Dragon magazine is incredibly useful and functional, it's also an arbitrary fix which even the author has to admit is subjective (look at the end of the second paragraph there). The truth is the rules offer up exceptions all throughout them without reference to other exceptions, and thus there really isn't an official answer about how to handle all the possible interactions. Lines of the rules arbitrarily impose conditions on other creatures that themselves arbitrarily impose conditions.
The table is one step towards 2e unifying everything but is on inspection massively surprising in its answers and certain to cause table arguments. I'm quite certain that a PC who has written on his character sheet "I'm only surprised 5% of the time" doesn't understand that to mean that sometimes he's surprised 62% of the time.
True enough, though I think most players can wrap their heads around an "under standard conditions" caveat.