I wonder what the life expectancy (such as it is) actually is for those zombies? If they are truly dead, then decomposition has to set in sometime. If the flesh is not maintained, it will break down by factors such as simple movement, temperature, accumulated damage (I assume they do not heal).
The body is over half water. That water must evaporate or start to liquify body parts eventually.
The bacteria which makes people into zombies exudes a barrier to protect it from the body's defences. This exudate is full of mutagenic DNA and shuts down the cardiovascular system. The rest of the cells are still alive but as you say, they begin to lose water and all of the normal bodily functions are suspended. After a certain amount of nutrients have been absorbed and if the host hasn't sustained too much damage the new DNA causes the release of hormones resulting in the smart zombies and the next phase of transformation.
The body is over half water. That water must evaporate or start to liquify body parts eventually.