I thought the Enterprise was the main character
Marsden and Bill are correct...especially in TOS, it is was much about something the ship "did" or "did not do", and/or about how the crew had to react when things went wrong...
The Lady Enterprise was and always will be the main character...
Note that Nimoy directed ST III. In the very film where he brought Spock back, he also took pains to kill off his rival for "main character" status, namely the Enterprise
The 1701-A then had the thankless task of filling in for a fan favorite, much like Priscilla Barnes replacing Suzanne Somers on Three's Company
or Cheryl Ladd coming into Charlie's Angels
for Farrah Fawcett. It's never quite the same, which was Nimoy's plan all along.