Yes, you can grow seeds from a hybrid. They will be more or less different than the hybrid depending on how different the parents of the hybrid were. In school we are taught that hybrids can't reproduce (horse x donkey = sterile mule), this is generally true with different species but with vegetables the different varieties are the same species so it is like two people having a child.
Growing out a hybrid will not get you back to the parents because each gene segregates independently of the others (more or less). It is like putting into a bag 100,000 pink balls, and 100,000 green balls, (the hybrid) shaking up the bag and then trying to pullout 100,000 pink balls.
Will an open pollinated variety degrade over time? Not on its own. The chance for a random mutation is about 1 in a million genes per generation, or one per one million seeds for a given gene.
The father of modern tomato breeding, Alexander Livingston, "discovered" that once a variety is stable it is that way forever as long as you keep other varieties away from it, so he contracted seed growers that would only grow his varieties in different fields/farms. It was also the way that he was able to select out new varieties and stabilize them by selecting seeds from from the best plants rather than saving seeds from the best fruits from differnt plants.
You can still buy most of these old varieties from Victory seeds and others.