The decline in the series is more due to the bundling than anything else. I wouldn't worry if I was Microsoft. They sell 2 million copies of Forza every year.
Although I would prefer if they release the games every three years instead of two.
Edit: I forgot to add the growing digital share. When you compare this gen Forza and last gen you have to take into account that a huge chunk of sales come from the digital store.
Non bundle copies of Forza games on the 360 are usually between 2 million and 2.8 million. What's the percentage of people who bought the bundles actually wanted Forza, that's something no one here can answer.