What do you think about All Our Yesterdays
as a potential replacement 'finale'? Sometimes I do think its hardcore exploration of the 'big three' as characters (seperating Kirk from the group and forcing McCoy and Spock into a survival situation) feels more fitting as a finale-of-sorts than Turnabout Intruder
, which as you say feels a lot like it could have been any old episode in the middle of the season.
I've seen the theory suggested elsewhere on TrekBBS that, as well as being something of a 'new beginning' of Star Trek, The Motion Picture
also feels a lot like the finale that TOS never got to have. Spock's character arc as seen throughout the original series (emotion vs logic) is basically wrapped up in a neat little bow in that movie.