From what little I know, it was the rise of regional cable TV in the states in the late 70s, that needed a lot of cheap filler programmes that kept Star Trek: TOS going*. Having 100+ shows instead of 70+ I don't think would make a whole lot of difference to how it was view**. That said we don't know what the episode would have been. If they were all stinkers, then it would have gone down as a turkey. If there were a few more gems. * It was Star Wars that started the movie franchise and their success made TNG and the rest possible. ** The horror channel in the UK has been showing TOS every day and has run through all of DS9 and most of Voy, so I think the love is still there.
The Original Series was a very strong formula that worked so well they mostly repeated many of the core concepts through the like of ST. I think it could have survived for another few seasons. It might have erred a bit toward getting repetitive with its storylines. There's a 10 year gap between the end of TOS and the first Motion Picture film so there's a good chunk of time in which they could have produced more stories and episodes without hitting the main films releases and such - even assuming that they'd have to end series development several years before 1979 to start making the film.
I'm sure the appeal would have been the same if Star Trek had gone a full five seasons. Maybe more so if they had been good ones. As Baylor points out, Roddenberry clearly had more in mind right from the start.