Incorrect: Larry Linville was offered a two year contract extension at the end of his 5 years, but he declined; it had nothing to do with the producers taking the series in another direction would not suit the Burns character.
True. Linville and Stevenson left to take roles in their own shows. Wayne Rogers left over money and the shift from co-star to second-banana. Burghoff just wanted to move-on. In all instances the producers took the departures as opportunities to shake-up the dynamics.
With respect to Maverick, the initial reason for bringing in Bart Maverick was as a hedge on the rising animosity between James Garner (Bret Maverick) and Warner Bros.Television as William T. Orr, executive-producer, was especially known for signing actors to onerous, one-sided contracts that led to many rebellions.