That’s a good point, my proposal isn’t to make most zombies evolve into epic boss battles, just a bit faster, tougher, and stronger as time goes on.
When you start and have a improvised weapon of some kind and no skill, single zombies are pretty easy, especially since you can kite them pretty easily, pairs and larger groups require steadily more work as the numbers increase.
Later on you have a decent weapon and have built up some skills, most zombies will still take a wack or three to take down, whereas if they hadn’t evolved, they’d be one-hits. (unevolved weak zombies will still be mixed in, they’ll go down very easily) Also they prevent a bit more challenge in that while they still move slower than you, if you’re a bit encumbered it might tip in the other direction.
Far down the road, you’ve got really good armor and weaponry and can dispatch ordinary zombies with ease, or even outright ignore some of them if you’re dealing with something more important, but more and more of them are specialized tougher or faster variants, or have special attacks, so you’ll have to deal with the ones that are actually a threat first.