If they knew it was a bad idea, why even bother entertaining the idea in the first place? Why add it to the Binder's lore at all if they didn't like it? They could have just written the 13th as some nebulous, unknown figure of prophecy (
like they originally did!) and left it at that. If they absolutely
had to incorporate his return into the final battle with Unicron at the end of the universe, they could have just left his identity ambiguous in the lore write-up, saying some "legendary figure
[identity to be determined]" would appear to fulfill his destiny in helping to destroy Unicron once and for all.
And like, I keep saying it, but the Last Autobot would have been perfect for this role. At the time of the Binder's first drafting, they could have even given the Last Autobot the name "Zeta Prime", since that name didn't first appear in TF fiction until Spotlight: Blurr in 2008, a year after work on the Binder lore first began. And while "Omega" is the actual last letter of the Greek Alphabet, the name "Omega Prime" had already long since been taken by someone else of greater notoriety at the time. To those less familiar with the Greek Alphabet, the name "Zeta Prime" sounds like it could work for someone who is the "last" of something since the name "Zeta" begins with a "Z", the last letter of the English Alphabet.
And if Hasbro
had gone with the Last Autobot being the 13th Prime, and had given him the name "Zeta Prime", IDW would have simply had to come up with some other name for their unseen Prime first mentioned in Spotlight: Blurr, which is no biggie.
In fact, the Zeta Prime from the WFC video game even shares a similar blue and gold color scheme to that of the Last Autobot, which makes this feel even more like such a tease.
And what's even more baffling is Archer's not thinking of the Last Autobot when, back when Vector Prime's toy was first released, so many people were comparing his design to the Last Autobot, claiming that Vector Prime's design was directly based on or inspired by that of the Last Autobot. For instance,
Ben Yee's review of the Galaxy Force figure from January 20, 2005 mentioned the similarity. And Archer was one of the two designers who came up with Vector Prime's appearance, the other being Eric Siebenaler. If Siebenaler was the one who modeled Vector Prime's look after the Last Autobot, I find it very difficult to believe that the subject never ever came up during any of their talks with each other in the toy's design process.