Wouldn't that work just as well or even better if it was hit points that advance slowly, rather than attack/defence bonuses? That a kobold can hit me even when I'm a 15th level fighter is irrelevant if it does 5 points of damage and I've got 150 hps. If I only have 20 hit points, then it's rather more significant. Increased defensive/offensive ability would still make the characters better, but a kobold would still be a a threat.
You could do that, but that's not what has been historically done (in D&D).
That goes to a basic over-engineering of the game system: Hit points represent that attacks are less effective. Attack bonus represents that hits are more likely. But, "hits" are a misnomer. A "hit" causes an expenditure of "plot budget", meaning, how long one can fight without assistance, and "hit points" are that budget.
That we have both is not entirely over-engineering, since we get two numbers to use as parameters. (A third number is AC.) That leads to a less linear model, with more interesting subtle features which also happens to match common ways of reasoning about fights. ("Common" does not necessarily mean "accurate in any real way".)
There are several tracks of related numbers: (AC, HP, AB), (DC, Save Bonus, HP), (Ability Bonus, Skill DC). For (AC, HP, AB), those can be adjusted as level increases say by increasing HP only, with AC and AB being modeled entirely by particular skill acquisition (better armor, a feat such as weapon focus), or they can be modeled with increases to (HP, AB), (3rd edition style), or could be modeled with increases to all three (4th edition style).
TomB