So...This is a bit of a conundrum in TWD universe.

We have very little in the way of walker mythology. The reason for this is that the show, and its source material the comic, have very little to do with the outbreak, a cure, or a fix for the problem. The zombocalypse is just a frame to answer the question "When the tapestry of civilization is gone, what becomes of humanity?".

However, we do have some insight to walker mythology. First, they do decay and become weaker over time. You mentioned their skulls splitting open and limbs being ripped off easily. In the comics, Eugene remarks how a walker they run across on the road to Alexandria is too weak to bite him but is still trying to lift its head. We have a few categories of zombies. "Walker/Roamer" being mobile wandering dead folks, typically alone or in small groups. There are also "Lurkers" that seem to be content just sitting or staying in place, and are fairly immobile. Walkers can be found in herds, which we have seen multiple times in both mediums. We also know that the zombies tend to be most sensitive to sound and maintain their sight fairly well across both mediums. Regardless of classification, all walker lack the ability to plan, organize, think, or anything requiring higher brain function.

Finally, you mentioned

But we've also seen walkers eat live horses and rip peoples insides with their hands like it were warm butter.

This has nothing to do with degradation or lack thereof. Season one and two's walkers have everything to do with show runner vision and nothing to do with mythology. Frank Darabount specifically wanted Romero style walkers (which is not founded in comic lore, just for the record) and that's what we see in season 1.