Firstly, you've always seemed like a decent dude to me and I respect the fact that, while we may have different political leanings, you've always been nothing less than respectful and generous to people posting here, and that you defend your positions with interity and calmness (comparing yourself to OSC is unfair to you, BTW). Secondly, do you have any insight into this question that was just asked (at least, as it applies to your design decisions with ACKS):
Thanks for the kind words. Likewise, I appreciate the generous goodwill you and others extend herein. And obviously I appreciate you doing this in-depth read through. It's actually quite exciting as a designer to see it unfold!
Now, on to your question. It was a deliberate decision, not just a throwback to B/X, but it's one I find myself changing my opinion on over time.
Let's acknowledge that the prevailing trend in modern design has been to make ability scores more important and class-and-level-based modifiers less important. 5E follows this to its logical conclusion with bounded accuracy for level-based bonuses and constant increases to ability scores over time. A 1st level thief with 16 DEX will move silently at +5 (+2 from proficiency, +3 from DEX). By 14th level with 18 DEX, he will sneak at +8 (+5 from proficiency, +4 from DEX). Thus the change is only 3 points in 13 levels, and ability score ranges from 80% to 120% of level-and-class based modifier throughout. If that same thief had DEX 10, he would sneak at +2 and +5 -- e.g. the average DEX thief at 14th level is only as good as a high school athlete (DEX 16 is only in the top 10% or so). Ability scores in 5E are absolutely crucial!!!
ACKS bucks the trend and makes level-based modifiers more important and ability scores less important. In ACKs, a 1st level thief will sneak at 17+ and a 14th level thief at 1+, a swing of 16 points - unrelated to ability score. This is a stunning difference in design outcome for two similar games.
So the first question we have to ask is: Why does ACKS deliberately keep ability scores less important than they are in modern games?
Part of the answer is thematic. The theme of the game is Adventurer - Conqueror - King. It is a game of becoming. It is a game of common men and women who rise to greatness. Aesthetically, then, an over-emphasis on ability scores would sends the wrong message about the theme of ACKS.
Part of the answer is aesthetic. It feels good to allow sub-optimal characters a mechanical opportunity to thrive. There are certainly human beings born with the greatest gifts in their field, and they show up in fantasy - Aragorn, Conan, etc - and in history. But there others who succeed despite the seeming deficit of talents. By making ability scores less important, it keeps characters randomly generated with fairly average ability scores quite playable. That, in turn, means that there is a greater diversity of the type of characters that can be effective to play. You can play a shaman with high STR and average WIS, or a thief with moderate DEX but great CHA, etc. Or, to use specific examples, in ACKS, it's easier to play someone like Taurus, the obese prince of thieves from Tower of the Elephant who nevertheless was a skilled climber and silent mover despite certainly not having a 16-18 DEX. Or Vincent, from Gattaca, who is inferior to his brother in every genetic measure, but the better man nonetheless. Or Audie Murphy, the hero of World War II, who was turned down four times for being too skinny to be a soldier.
These two answers, however, don't fully respond to the question. Since ACKS provides generous benefits from leveling, the modest ability score modifiers would not necessarily swamp the value of training. A 14th level thief with DEX 9 would still be 1+, while a 1st level thief with DEX 18 would be 14+, so it's still a tremendous difference. Moreover, ACKS does allow ability score modifiers to apply some of the time, such as in to hit rolls, damage rolls, AC, initiative, open doors checks, and so on, so the answer is over-explanatory in that it suggests other parts of the game are designed wrong.
The next layer of the answer is that ACKS was deliberately structured so that some classes are more impacted by ability scores than others. Fighters, for instance, have all of their important game mechanics affected by their ability scores - to hit, damage, AC, hit points. Clerics, on the other hand, have virtually none of their class effecs impacted - turn undead, divine repertoire, and divine spells per day aren't affected by Wisdom. Mages and thieves are in between. This, again, allows for a much wider variety of fun, playable characters.
However, over time I've come to feel that this latter choice wasn't necessarily a good one. Among other things, it has lead to Wisdom being a dump-stat in ACKS, and it has created an ugly asymmetry in that some target values that improve over time are affected by ability score modifiers (attack throws, magic research throws) while others are not.
In my most recent home campaigns, I have had the Wisdom ability score modifier apply to ALL saving throws. In the Heroic Fantasy Handbook, where the emphasis is on more heroic characters, I have been toying with the idea of offering some rules to incorporate more ability score modifiers on rolls.
Finally, this wouldn't be an ACKS essay without mention of simulation. It is an open question within the field of human achievement how much talent matters and how much training matters. As far as I can gather (and I've read a lot, though not by any means exhaustively) the current findings are: (a) a minimum floor of talent needed to pursue training at all, (b) very high returns from talent early on with a point of diminishing returns and (c) continuous improvement from training over time IF training is properly done with feedback from failure. So for instance older surgeons are typically better than younger surgeons, even though the latter have nimbler fingers, because the older surgeons' fingers are *nimble enough* and they have years of training. And I think ACKS' approach models these realities quite well. Games like Cyberpunk 2020 or 5E, where half the bonus is from ability scores, get it wrong; talent is not half the battle. Talent lets you train better, faster, and to higher aptitudes, but training and practice is what matters.
Character classes have prime requisites that set a minimum floor. Characters get a bonus to XP from high prime requisites to represent the higher return from talent. And characters get a bonus to starting proficiencies from high INT. I suppose if one wanted to be really realistic, probably the best way to handle it would be to have ability scores determine the speed at which leveling of skills occurs on a skill-by-skill basis, but that's more crunchy than ACKS can handle, and probably more crunchy than anyone would want.