I am a strong advocate of adding a few more classes, and here's an analogy to support that:
D&D is a plant, a tree for example. This tree needs pruning with errata, sunlight from money that the players give for the hobby, to breathe fresh air, that is the new settings and adventures, and a solid foundation in the ground that is legacy, tradition, and history of D&D. The crunch (rules, classes, races) are the water that keeps the tree alive. D&D needs all of these aspects, but too much of any of them will kill the tree. Too many errata will cause D&D's rules to be fluid and unknown, causing more and more debates of the rules, which will cut away too many branches of the tree. Too much money, and WotC is suddenly only making rules to grab cash, which will scorch the tree. Too many settings and adventures, and they will stop feeling unique and distinct. Too hard a foundation, and the tree is stuck in stone and starved of ever changing or growing. Too much water, and you will drown the hobby with bloated rules, too many classes, and too many options for the races.
But, there is another aspect of this. Too little errata, and the broken aspects will be set in stone, and the hobby/tree can't grow properly. Too little money, and the company goes bankrupt, and the hobby/tree starves. Not enough settings and adventures, and the hobby/tree is suffocated by lack of ideas and concepts. Not enough of a solid foundation, and the tree will be blown over and die by no continuity in the hobby.
But, most importantly to this discussion, D&D needs water. Not too much, or the game/tree will be drowned by content and rot away, and have a new edition take its place. Too little, the hobby/tree is killed by thirst, for lack of content (classes, races, races).