This is true, but even now in its current run it's gone for six seasons, with another one upon us at the end of the year (plus another season still if you count the final Tennant movies as a season).
There's a lot to be said for a show that can totally reinvent itself with each new incarnation - a new face, new mannerisms, a blank canvas as you say, which ultimately give it great longevity. The only other show that I could say something similar about would be the 90s version of The Outer Limits - new guest stars each week, and new stories. More often than not, I enjoyed that show too.
