The problem as I see it is that even in AD&D we abstracted sufficiently and use terminology that obfuscates what we're trying to communicate.
We see things like "Armor Class 5" or "Armor Class -2" and what we've abstracted away is the notion that "Magic Plate and Shield is First Class Armor" (i.e., AC 1), and things after that are "Second Class" or "Third Class" (or "Lesser Class").
The fact that Armor Class can go into negative numbers means we overthought the system. Now it's just a mathematical construct instead of what it was originally intended to do...
"Best Armor" = First Class (AC 1)
"Really Good Armor" = Second Class (AC 2)
"Good Armor" = Third Class (AC 3)
... and so on.
Since we're no longer using AC 1 as shorthand for "the best armor" it now becomes the question of which is the easier mathematical operation:
ASCENDING AC: Start with a hit roll. Sum all the bonuses that may apply due to magic, high ability scores, Proficiency/Base Attack/Bonus etc. Compare this final value with the armor class of the defender. If the final result is greater than or equal to the AC of the defender, the attack hits.
DESCENDING AC: Start with a hit roll. Sum all the bonuses that may apply due to magic, high ability score, etc. Subtract from this result the THAC0 value of the attacker. Now add the armor class value of the defender (which for negative AC really means subtraction). Compare this final value with zero. If the final result is greater than zero, the attack hits.
One of these has a lot more steps, including multiple subtraction steps. I think using Ascending AC is a lot cleaner. To say nothing of the counterintuitiveness of magic armor and shields in the Descending AC paradigm LOWERING your armor class by the amount of their "plus" (e.g., "take a fighter in plate mail with a shield... he has an AC of 2. Now hand him a +3 shield. His AC is now -1. Yup, 2 plus 3 equals minus 1.")