Would you feel differently if he had proposed replacing Friedman permanently?
I don't care who they replace. I don't think any of the columnists are indispensible, and rotating seems like a way to keep the pages fresh and relevant.
I also think the pressure of producing two relatively short columns a week degrades the writing ability of the columnists. Compare Krugman (whose columns I still generally like) before and after he started his NYT gig.