In 4th Edition, that's easy to answer. Monster defenses scale smoothly by +1 per level. Character attacks scale by approximately +1 per level, though not as smoothly as monster defenses. Still, the relationship is tight and balanced.
In AD&D, the relationship is complex. Character attacks (for fighters) scale at approximately +1 per level or a fraction more. But monster defenses are all over the board. Here's a plot of AC for all monsters from the AD&D Monster Manual that have 20 or fewer hit dice.
|Scatter Plot of AD&D Monster AC by Hit Dice|
What's important about the plot is the slope of the approximated line when compared to the slope of the attack bonuses. At 1 hit die, the average monster AC is between 6 and 7. At 20 hit dice, the average monster AC is 0. That's a spread of only 7 points. Yet the AD&D fighter's* attack bonus increases by 20 or more points between level 1 and 20. That's a huge discrepancy.
|AD&D Fighter Attack Bonuses|
* (By comparison, the cleric's attack bonus increased by 11-15, the thief's by 10-14, and the magic-user's by 9-13. The range depends on how much of a bonus the character gets from a magic weapon, which could vary from 0 to +4. I assume any +5 weapon was given to the fighter.)
So in AD&D, as characters advance up the level scale, they constantly gain ground against the monsters' defenses. A 15th-level fighter doesn't just hit lower-level monsters more often; he hits all monsters, even those of his own level, more reliably than before.
Before anyone gets too excited, I need to point out another big facet of AD&D: the damage that characters do with their weapons doesn't scale at all across levels, aside from magical bonuses. Heroes' attacks need to get more accurate because hitting more often is the main way the characters do more damage to the monsters, whose hit points increase in direct proportion to their hit dice.
The last conclusion I want anyone to draw from this analysis is that edition Y got it right and edition Z got it wrong. My goal is to establish some facts so we can talk about these subjects from a factual basis instead of from the vague and emotional impressions we form during the heat of play and across the haze of years. Early editions of D&D handled math very differently from later editions. I'm not about to pick a winner--not yet, anyway. That's a subject for Friday.