It's trivial to show that natural selection always leads to things getting worse? That's certainly wrong. If that isn't what you mean, than I can't guess what it might be. Natural selection always reduces diversity? That's not true either, but it is true that selection often reduces diversity. I'm really having a tough time discerning your argument here.
This is really hard for me to explain without resorting to pictures since it is in pictures that it the easiest to explain.
Still, again I shall use the genetic programming example since it is the easiest to talk about and, in this case, it does directly translate to biology.
Let's take a population of "Super Frogs". Now these super frogs have the potential with all the information in them (characteristics: colour, shape, size, etc) to specialise to be any "kind" of frog. Kind of like the "common ancestor of frogs".
Draw a circle to represent this population. Call it A.
Draw another circle with a dotted line encompassing all of A. We'll call it A'
This outer circle, A', is the potential reach of A from the current population of A. That is, it defines the reach of the possible population with mutations, breeding etc.
Now, logically, as these super frogs reproduce etc, the population becomes specialised to area. Some frogs only survive if they are large, others if they are small, others again if they are green, others if they swim fast. Whatever the reason, different features of the population A are 'bred out' due to being unfit.
Draw these four circles within the A circle - again extend by drawing dotted lines around them.
It is completely normal, and indeed expected, that the area covered by A and A' significantly outweighs the area covered by the small specialised populations. The combinatory possibilities have been reduced. Multiply this by a large number of generations and you have an instant problem. Diversification is supposedly increased - you have a large number of different types of frogs - but in actual fact genetic potential has been lost.
The whole design of genetic programming was built around this logical principle. Otherwise "natural selection" would be a complete waste of time and we certainly would not be able to solve software problems with it.
This IS happening with every population round the world. We are getting greater "specialisation", but at the cost of lower information potential. Kinda like kinetic versus potential energy.
Why do you think mongrels have less disease problems than particular breeds? It is purely because the information loss is not the same. There is less likelihood that the information necessary to combat disease X has been bred out of them.