http://new3.fjcdn.com... in my opinion... I think it has been on a decline since Need for Speed: Carbon.
NFS: ProStreet is an awful game. NFS: Undercover is basically sonic 2006 (it wasn't finished). NFS: Shift 1 & NFS: Shift 2 are boring and have nothing to do with need for speed. NFS: The Run is very, very bad. The only good game that came out after NFS: Most Wanted is NFS: Hot Pursuit (2010).
I agree with you that some of the NFS games did lose the brand's important factors during 2006~2009. But NFS is getting more popular and fun since 2012, such as the Need For Speed: Hot Pursuit, Need For Speed: Most Wanted 2012 and Need For Speed: Rivals became one of the best NFS games I've encountered. It also did increase in sales since 2010 after NFS:HP came out, so I disagree with you, even if 2006~2009 NFS games were bad, I think it's actually doing pretty well now.
Another thing that I just want to say is that the chart that you put on looks like someone's point of view of the quality of NF game overtime. There isn't any official watermarks or anything to prove that the chart is criticized by an professional gaming review company or anything. Also, I don't see how the quality is still declining after 2010, NFS boomed after 2010 with awesome reviews about their games being awesome and how well they're made. So if you can't prove that the chart is from an professional gaming review, such as IGN, I will keep going.