I recently read this Destructiod article detailing why Sakaguchi decided to call the game Final Fantasy.
The article states that he wanted to use the abbreviation "FF" because it rolls off the tongue nicely in Japanese.
According to Sakaguchi, the team wanted a title that had a simple abbreviation in the Roman alphabet (FF) and a four-syllable abbreviated Japanese pronunciation ("efu efu"). Because of the setting and style, "fantasy" was an easy choice. "Final" wasn't the first choice of adjective, however.
The initial proposed name was "Fighting Fantasy," but it was already taken by a tabletop game. And so it is, Final Fantasy.
"To be sure, we had our backs to the wall when we were developing Final Fantasy," Sakaguchi said, "but really, anything that started with an F would have been fine for the title."
The Destructoid article links to a Famitsu article (in Japanese) where Sakaguchi says this. The article is dated 24th May 2015.
The article goes on to explain that FF composer Nobuo Uematsu confirms the story of "Final implies it may have been the last game" but that was in a Wired article from six years ago.
This doesn't disprove the original theory that "Final" was the last one. But it's interesting that Sakaguchi doesn't appear bothered about what the first 'F' stood for, but never denied the theory that they made it 'Final' because he was planning on quitting if the game was not successful.