There are two different sides to this argument. The first of these comes from the Developers and Producers. Thier argument lies in the fact that the vast majority of consumers want these rehashes. This, as we can see, is true. The best selling games after all are just those. The Call of Duty series is one of the best selling of all time, and, from the second one on, they may have made minor changes, but the core game remains the same. This is what sells. They also argue that when they turn out an original title, say, psychonauts, one of my favorite games of all time, barely anyone buys it, but it is an amazing and innovative title. The studios argue that when they put the time into turning out a new IP, nobody buys it. This also seems to be true, most new IP's, unless they are artificially boosted, say as in a bundle with the sale of a console, are somewhat poorly selling as compared to other big name titles
The second side of the argument comes from the consumers. Thier argument lies in the fact that the boards of large companies (Kotick I'm looking at you) dont want to take the risk of change, they dont want to risk a failure of a game, something that many companies have before experienced, and as such, they stay with what is safe, they may refine the experience for a sequel, but the core of the game stays the same.
Thus, I believe, the real innovation must come from the indie guys, with the small, somewhat nonexistant budget, they are able to take risks, and if they fail, it is of no consequence to shareholders of whom the large companies are responsible to turn a profit to. The diamond in the rough example of this is of course Markus Persson, or as he is better known, notch. His game, minecraft, as you know, is a multi million dollar runaway success, It is an incredibly innovative game, the likes of which we have never seen in the industry before. Thus, the way forward is looking for the trials and tribulations of indie games, and picking out the gems