Keeping all types of enemies pertinent throughout, I think, is a mark of good design.
The challenge that new enemies presents to the player should have more do with the complexity and/or number of decisions that they must make to solve the problem. It shouldn´t be about numbers unless there are opportunity costs.
So, rats, for example, may deal more damage or be harder to hit when the user is wearing Plate Mail or wielding a Great Sword. Because we´re in an area where the diversity of enemies would demand Plate Mail, Rats are now a peculiar challenge. However, in earlier levels, where there are ONLY rats, we don´t even have to think about the equipment we´re using, we just pick the best for the job. Perhaps, instead, rats deal more damage the more rats are adjacent to you and now we´re in a wide open space... there are many ways to allow the rules of the game to weave greater complexity without messing with numbers.
Furthering the vein of complexity, the challenge should emerge from interesting configurations of enemies. Certain enemies should converge to produce a challenge that is orders of magnitude greater than what either provided separately or what increased numbers would provide.