Guess what? The DMG explicitly tells you to break out of that range if it's warranted, though it warns you that doing so can have consequences you should prepare for.
It even explicitly gives reasons why someone would do something at +8.
Meaning, you could just as easily simply fill the world with encounters of predefined level, and let the players have at it. The rules wouldn't be any different, the game wouldn't play any different, and players would in fact feel the progression.
In other words, if you have a world where everything is in perfect lockstep with the characters, it's because you chose to make it that way. The books tell you NOT to.