I figured they would've mostly just given his material to Rollin. Landau, despite being only a "special guest star," was effectively the series lead by the end of the season. The only reason it didn't stay that way was because he didn't want to commit to the series for more than a season at a time, so the producers brought in a new lead rather than just promoting Landau to the lead. I recently rewatched the M:I movies (upon finishing the revival series), and it struck me how the so-called "Jim Phelps" that Jon Voight played in the movie was really more like Dan Briggs. Not only did he have a stern, aloof quality that was more like Briggs, but he supervised his team from the safe house rather than actually joining them on the operation, much like Briggs did in the later-filmed episodes. The DVDs are in broadcast order. Maybe they developed the script before they'd decided to bring in a new lead rather than making Landau the star, or before they'd cast the new lead. Not to mention a score by TOS composer Gerald Fried. Oh, absolutely. As I often say, M:I was really a con-artist/heist show rather than a spy show; the spy stuff was just an excuse for the con artists and thieves to be the good guys.