Gamasutra has published a four-page article entitled "The Circle of Life: An Analysis of the Game Product Lifecycle", in which they discuss how video game genre life cycles have changed over the years. The article primarily focuses on the massive decline in the number of adventure games being developed, but its message certainly applies to specific types of role-playing games as well (single player party-based RPGs, for example).
Like most product categories, genres are ultimately defined by the customers. A customer purchases a random game and discovers that it fits their entertainment needs nicely. Since most individual games have a relatively short burnout period, the player returns to the store seeking another similar game. Often, they'll find that the sequel is not yet available so instead they pick up a similar title.
Occasionally, they'll come across a game with the same brand, but with a different set of mechanics. Perhaps someone decided to make a Kings Quest 3D action game instead of an adventure game. The customers are often disappointed. For many game players, brand alone was not a meaningful measure of value.
I find the strong connection between game mechanics and product categories quite fascinating. A very profitable segment of customers believes, despite all our effort spent on innovation, branding, packaging and licenses, that similar game mechanics are the defining characteristic of gaming value.