Okay, let's take a crack at this.
Making it Spock's brain that gets removed makes it more understandable that he can survive the operation, having Vulcan anatomy which could have any number of characteristics that Humans don't have, so we can understand it's likely why he manages to live until he can be sustained by tech. Also, the technique is informed by alien tech.
Further, removing Spock from the equation temporarily means the crew believes they have to figure out what happened without his insight to guide them (until they find the machine his brain's connected to). There's a time limit to retrieve him, so the clock's ticking and they're significantly hampered by his mental absence.
Removing anyone else's brain: I suppose the person would be declared dead and a funeral would be held, or perhaps delayed because the crew feels the dead man should be buried with his brain, at the very least. Then when they find there's a chance to restore the dead person, they'd jump at the chance. If it was Kirk, that would leave Spock in command. It couldn't be McCoy's because you'd need another doctor to perform the operation, so a guest star.
One thing this episode does is it requires Spock and McCoy to work together to save Spock's life. They complement each other.
Making it Spock's brain that gets removed makes it more understandable that he can survive the operation, having Vulcan anatomy which could have any number of characteristics that Humans don't have, so we can understand it's likely why he manages to live until he can be sustained by tech. Also, the technique is informed by alien tech.
Further, removing Spock from the equation temporarily means the crew believes they have to figure out what happened without his insight to guide them (until they find the machine his brain's connected to). There's a time limit to retrieve him, so the clock's ticking and they're significantly hampered by his mental absence.
Removing anyone else's brain: I suppose the person would be declared dead and a funeral would be held, or perhaps delayed because the crew feels the dead man should be buried with his brain, at the very least. Then when they find there's a chance to restore the dead person, they'd jump at the chance. If it was Kirk, that would leave Spock in command. It couldn't be McCoy's because you'd need another doctor to perform the operation, so a guest star.
One thing this episode does is it requires Spock and McCoy to work together to save Spock's life. They complement each other.