Brian Steinberg Senior TV Editor It’s an idea that would make any news executive salivate. What if MSNBC revived CNN’s old “Crossfire” show, enlisting Tucker Carlson from the right and Don Lemon from the left? Such a concept would normally be unthinkable. Tucker Carlson is known for his work on Fox News. Don Lemon has long held forth for CNN. It has been that way for years. Until, as of earlier this week, it wasn’t. Within the space of 90 minutes on April 24, Carlson was ousted from Fox News Channel and Lemon was pushed from CNN. Both were stalwart personalities for their networks. Both were valued for their ability to spark conversation, debate, pushback and even furor. For years, that has been the cable-news coin of the realm. As the economics of TV shift, however, it is becoming clearer that media executives may have less patience for TV-news pot-stirrers.