Maybe if you add in all the letters that Paramount and the various local stations got in with the 10,000 that NBC got, it might get closer to that one million figure.
Closer to the 100,000 figure from the
TV Guide letter, perhaps. But I think the preponderance of evidence is that the "million letters" claim was just typical Roddenberry bluster.
Precisely what I was thinking! I feel that if the show did finish it's 5th season, it would have garnered the whole "this show has gone to shit" attitude that some shows get after stretching (diluting) their idea out too much. Some shows are AMAZING for a season or two, then they complicate plots too much and it gets boring/cliche/stupid.
Well, again, let's keep in mind the difference between '60s/'70s TV and modern TV. Back then there was no serialization except in daytime soaps, so the plots in later seasons wouldn't have gotten "complicated" in the way I assume you're suggesting. Each episode would've still just been a completely self-contained entity with little or no reference to anything before it and little or no impact on anything after it.
Certainly '60s and '70s shows did tend to go downhill in quality as they aged, but it wasn't due to plots becoming more complicated, more like they became lazier or simpler-minded or more repetitive. There was often network pressure, particularly on genre shows, to dumb things down or go more for the lowest common denominator. Or the writing staff changed and the writing grew weaker or less authentic to the premise and characters.
(Or sometimes it got worse for no discernible reason. The third season of
Batman was deeply lame and stupid compared to the first two, with only the introduction of Batgirl as a saving grace, yet it had the same writers and producers as before. It gave up the 2-part cliffhanger-based format of the first two seasons due to a scheduling change, so that may have thrown off the writers' rhythms, but that alone wouldn't have explained how much the quality plummeted. Although really, the quality was already dropping off in late season 2.)