I am a 26-year old American, and I have never seen anything other than nuWho.
There's absolutely nothing wrong with switching out the companions, but it's certainly not necessary to keep things fresh. There are other things you can do to accomplish that.
I just...well...I like Amy and Rory! I liked Donna! I liked Rose! I didn't want to see them go!
That's all it boils down to. I like these characters, and I don't like the assumption that they're only worthwhile for 2-3 seasons.
There's absolutely nothing wrong with switching out the companions, but it's certainly not necessary to keep things fresh. There are other things you can do to accomplish that.
I just...well...I like Amy and Rory! I liked Donna! I liked Rose! I didn't want to see them go!
That's all it boils down to. I like these characters, and I don't like the assumption that they're only worthwhile for 2-3 seasons.