I disagree with a number of points here. The first being, bows and hurled missiles could be made to use STR bonus for damage (basically accounting for the "bow that only a Strong Hero can string and draw" type of trope), but issues of cost and availability are left to the DM, so let's set that aside for now, and look at our simple fighter with a DEX 10.
My recollection is that, in Gygax's DMG, the option of "STR bows" and the like is presented as just that - an option. And I don't remember seeing any instantiation of it in any module (eg in NPC damage numbers).
his raw to-hit numbers quickly outstrip a Thief or Assassin's DEX-enhanced to-hit. More importantly, the only ranged weapons allowed to other classes are the dart (everybody), the sling (druids and thieves), crossbows (monks), and thrown daggers (everybody but clerics) and hand-axes (monks again). Only the Fighter and the Assassin have access to bows, and the fighter's to-hit rapidly outpaces even a DEX 18 assassin's. Nor does a Fighter especially miss out by not pumping DEX, since the fighter's getting at best a mere +2 to-hit bonus with high-end Exceptional Strength. And while a bow does only 1d6 damage with no damage bonus (not even for the DEX 18 Assassin, since in AD&D Dex didn't provide a damage bonus), it has a fire rate of 2 per round. So ranged combat is not really a step down from fighters' melee shenanigans.
The question was not whether the fighter is a better ranged combatant than the cleric - which is true in 4e also - the fighter is proficient in bows and heavier thrown weapons while the cleric is not). The comparison is of fighter ranged attacks to fighter melee attacks.
I don't really agree with your comparisons of fighters to thieves, though. A 5th level fighter has a THACO of 16. A thief with the same number of XP is 6th level, and has a THACO of 19. With a DEX of 17 the thief hits as well in missile combat as the fighter does; the fighter has not outstripped the thief.
A 9th level thief needs slightly fewer XP than an 8th level fighter, and at that point the gap closes a little - the fighter has a THACO of 14 (8th level THACO is the same as 7th unless an optional rule is being used), and the thief a THACO of 16. So a 16 DEX will close the gap. (DEX 17 if the optional rule for fighter to-hit is being used.)
An assassin needs more XP than a thief, but even then the gap doesn't quickly open up beyond 4: a 9th level assassin needs slightly fewer XP than a 9th level fighter, and the THACO is 16 compared to 12; and a 13th level assassin, with the same XP as an 11th level fighter, has a THACO of 14 compared to 10.
And in UA a thief can use a shortbow.
Comparing a fighter's damage with a bow to melee damage: 2d6 averages 7, compared to (at 1st level) 3/2 * 7.5 (for 1d8+3 from 16 STR and specialisation), which is greater than 11, or more than 50% better damage. If the fighter has 18/01 STR, the melee damage is 3/2*9.5 (for 1d8+5), or greater than 14, which is more than double the bow damage. And that's before allowing for the +2 to hit (+1 STR, +1 specialisation).
Even without UA, the longsword does more damage with 16 STR (7.5 vs 7) and noticeably more with 18/01 STR (9.5 vs 7). And that's before we get to size L creatures do, where the longsword steps up to 1d12. And the +1 to hit also increases damage, especially at lower levels where base chances to hit are often less than 50%.
Once magic items come into play the gap opens up, as magic bows in the DMG don't go above +1, and magic arrows tend not to be reusable.
I think that missile combat is a step down for the fighter, especially in UA (assuming melee weapon specialisation).
I have seen some fantastic archers and other missile-specialized characters in AD&D. Darts are so powerful, with rate of fire 3 and a high strength bonus, I had to change that.
The STR bonus may be a core rule in AD&D 2nd ed (I don't know), but is not in AD&D 1st ed.
As for bow specialists, yes they are strong. But my comparison was of a melee-oriented fighter's melee attacks compared to his/her missile attacks.
EDIT: My thief DEX numbers are out by one - so DEX 16 should be 17, and DEX 17 should read 18. And once the assassin gap opens by 4, the fighter has a +1 to hit advantage over the 18 DEX assassin.