Don't foret that Angry Birds was Rovio's 40th game. Stickability counts.
I think to succeed though you need to put out more than 1 per year. I also think there is likely to be a network effect, if someone likes one of your games they may try the rest... this is the equivalent of the McDonalds "you want fries with that?" - that is they know they only get a certain number of people through their door, so they need to extract as much value from each customer as possible.
You will only get so many eyeballs, so if you have more than one game you may get people who try one also trying the other - free marketing. :D
I think to succeed though you need to put out more than 1 per year. I also think there is likely to be a network effect, if someone likes one of your games they may try the rest... this is the equivalent of the McDonalds "you want fries with that?" - that is they know they only get a certain number of people through their door, so they need to extract as much value from each customer as possible.
You will only get so many eyeballs, so if you have more than one game you may get people who try one also trying the other - free marketing. :D