I agree with Starkers
. The RTD era was getting a bit repetitive. While the brilliantness of Tennant probably could've covered for that for awhile, it would've eventually gotten old.
The switch to Moffat/Smith really prevented the continuation of that process. Generally I enjoy the current state of the show but it's not perfect. And like RTD's era, a certain repetitiveness is creeping in.
However, I think this team has been mostly brilliant. And, IIRC, the audience share has remained fairly stable, it's just that fewer people are watching overall.