We're pretty much debating macguffins here, but seeing as N.E.S.T. had little trouble keeping Decepticons pretty well in check (and the human military handled themselves pretty well in egypt/jordan), the Fallen's forces seem much less dangerous without the ability to destroy the entire solar system. I guess there's also the idea of the Nemesis and the hatchlings, but that's something that wasn't really given much weight in ROTF or DOTM. The "only a prime can stop a prime" thing was interesting, and I suppose it would necessitate the ressurrection of Optimus, but it clashes with the theme of "personal choice" in the Cybertronian conflict we see, and isn't given much thought in ROTF, especially given the Fallen really only does things at the very end of the film.
If Optimus has been revived with the shard Mikaela had, then Sam would've been captured by the government and turned over to The Fallen and the sun would explode.
No Megatron would have hunted Sam down, cut out his brain to let the symbols lead him to the matrix, making the Fallen still a threat. It was a race to get to the matrix before they could get to Sam, they needed Jetfire for that.
Was there any indication the Fallen or the Decepticons would have had more luck piecing the knowledge in his head together, or making sense of it? It seemed the final location would have required a pretty good understanding of earth history and culture, which they didn't appear to have until DOTM. Public discussion of moderator decisions is not allowed. Suffice to say, she received too many infractions, I guess?
Jetfire or the Fallen would have been needed to translate the symbols, so yes there is an indication that the decepticons would have more "luck" on piecing the knowledge together. The location of the matrix was based on stars and geography. The Fallen had to have had some familiarity with the geography of the planet, especially over the thousands of years since his arrival.
