A movie about TNG movie wouldn't have enough conflict and there's no defined endpoint. The writers end up making a better series despite Gene Roddenberry. That's not interesting enough unless you want to be a writer and want to know what it's like to deal with a difficult boss. But that could be about anything, it wouldn't have to be about TNG. And by the time TNG ended it was just a piece of the large Star Trek puzzle so you could take that story all the way up to 2005.
TOS has well-known conflict and on multiple levels. Competition between the actors, Desilu strugging to get the series produced on-time and on-budget, the fights with NBC, William Shatner's pranks, Gene Roddenberry's shennanigans, the Save Star Trek Campaign, the third season deathslot, and the crumbling of morale during that year. There's also a clearly defined beginning, middle, and end from 1964 to 1969, with only one production and nothing overlapping or continuing on immediately afterward. It also helps that Mad Men has made this time period more marketable and it would be very similar. Just change advertising to TV production.
A movie about TOS is definitely the way to go if you were going to do something like this.
