There are still the codes of interplanetary conflict that most of the Transformers (Autobot and Decepticon alike) previously appeared to value, and violation of those rules meant that an agent of the Tyrest Accord, like Ultra Magnus, would be hunting them down. I'm hoping for some explanation as to why none of the Decepticons are even batting an eye anymore at the thought of open conflict, like they were when Megs first called Sixshot to Earth.
Admittedly, it's been a bit since I've read
Devastation, and I don't remember a lot of
Revelations and have the sinking suspicion I missed an issue or otherwise did not read it, but:
Ultra Magnus pretty much threw his lot in with the Autobots in
Revelations, I thought, and haven't we seen him once in
All Hail Megatron? Maybe I'm crossing stories. Not that any of this means much, and Ultra Magnus isn't the only one, but even if they did hunt for Megatron, they'd be effectively hunting the entire Decepticon force, and I don't think I'd envy them that.
As for the Decepticons and open combat, which ones are left from Starscream's original force? Runabout and Runamuck got offed at the end of
Devastation. Did Astrotrain? They were primary amongst the worried ones, and even then, a lot of the worry stemmed also from the fact that Starscream was less megalomaniacal than was Megatron. Even so, when Starscream came back into play, he proved himself capable of downing Sixshot. If Starscream throws his hand in with Megatron's (which you have to think he would, both for the power play and to show himself as loyal to the guy who kicked his butt some issues back) - who are the other Decepticons to any longer worry?
There also doesn't seem to be much in the way of explaining why Earth is so important anymore, since McCarthy has said that ultra-energon won't play a role in the book; as well as a story behind why the Autobots don't have the Magnificence anymore, which could have easily tipped the war in their favor, even without Prime; and not to mention how and why Cybertron is now perfectly inhabitable. I mean, I'm hoping that McCarthy will explain these inconsistencies to the reader, but with the way the story is currently going, I can't shake the feeling that the most explanation that we'll get is something along the lines of some offhand remark from Hot Rod that "oh, yeah, we lost the Magnificence," or Wheeljack saying "[one line of technobabble], and now our planet is safe to run around on once more," and then never mention the various incongruities ever again.
This is probably fair, but it then becomes a matter of what one expects or wants to see vs. what is actually integral to the story.
All Hail Megatron can remain a standalone and complete story without explaining what makes Earth worth taking over, what happened to Cybertron, what happened to the Magnificence, etc. Honestly, all
All Hail Megatron was slated to be is the campaign by the Decepticons to rip Earth a new hole and take over - for all I don't want to call it one, essentially a summer popcorn blockbuster flick. His use of the Autobots stranded on Cybertron is justified by answering the where they are and why they're not defending Earth, following their retreat from the planet per Prime's orders. But the mini could use no Autobots at all, and still remain complete. Shorter, maybe better, but that's something to be judged at the end.
Will we see things explained? Eh, maybe. I think we'll see more about Cybertron, as well as what happened to Prime. Will we see any talk about what makes Earth worth it? Probably not. I don't think it would necessarily fit into the mini, one, and two, this portrayal of Megatron (through all the titles) hasn't been one of "military genius." Especially since his coming to Earth - in Furman's work - it's been more of flexing his power and crushing opposition in his hand. Why up and leave Earth? It's a planet for the taking, and they're there. And for the trouble he went through during
Escalation and
Devastation, why up and leave the way his nemeses - the Autobots - did? Then again, we might eventually see the answer to your question.
All I'm saying is if, at the end of the day, McCarthy's premise for the story and his orders from editorial were to jump forward some amount of time and, from that point, move forward with where the universe was / would be, then
All Hail Megatron, to the issue it's currently at, has accomplished and is accomplishing that task. For we readers, would a line or two here or there about why this is or what that is be nice? Sure, and convenient. Is it McCarthy's job? Not necessarily, and especially not if it would be spoiling ground that has yet to be covered in work meant to "catch up" to
All Hail Megatron. To be left wanting is generally better than encountering something redundant.