The Overlords did join the Overmind.
Just not the ones who serve the Overmind. As Karellen said they have become as advanced as they can and can go no further. The reason is But What About the Astronauts? See, Jan (book)/Milo (series) did not die with the rest of humanity because he was not on Earth. The Overlords on their home planet ascended to join the Overmind, but the ones on an offworld colony weren't there. You have to be on your home planet to ascend. The Overlords who joined the Overmind are the ones directing their distant relatives to do the Overmind's bidding. So the reason their "technology led them astray" was you need to join the Overmind before leaving your home planet.- Relatedly in the TV mini-series, if Rachel had lived, then she and Milo probably would have taken Karellen's offer to leave with the Overlords.
The "Devil" character in the Cantina in Star Wars is an Overlord.
The one here.
Before they started working for the Overmind. Heck, maybe the Overmind is The Force! Maybe the Overmind already cleared out that far away galaxy a long time ago after the Force was balanced! Maybe that's the only way to stop the Dark Side - not give anyone the chance to create a galactic civilization.- Doesn't work. Devaronians don't have wings or tails.
- Yeah, but we don't see that in the film universe.
Overmind is actually a parasyte.
Many parasitic wasps tend to find victims, immobilize them and placing larvae in them, which then grows and consumes its host. If you look at the event from this angle, Overmind might just breed it's offspring to expand itself, using civilizations as hosts. Overlords are there to help pacify and de-fang the victim civilization, so it couldn't resist, and wouldn't even wanted to. So there's no "ascension to a higher plane of existence", only murder. The Overlords directly state that, saying that "these children aren't yours". The Overlord's are stuck because their fundamental decency makes them undesirable to the Overmind
Related somewhat to the above, there's no reason to assume the Overmind to be benevolent. What if the reason the Overlords haven't been able to join the Overmind is because the Overmind don't want a decent species to join them? The Overlords seemed to genuinely care for humanity and other species too (e.g. they can't stand animal cruelty). The Overmind probably want species they consider like-minded to join them (since, if they are one mind, any influence from that species won't change who they are). If the Overmind really are bad guys, they may consider a species like humanity (suggested multiple times in this novel may blow itself up if left to its own devices) to be ideal, whereas a species like the Overlords (actually pretty nice guys) not to be suitable. If so, it's possible that the reason why some "stagnate" whereas some go through the process of joining the Overmind, is that the Overmind selects the species they consider suitable and start the process in some way. The Overlords, as basically a compassionate species, may be ideal for ensuring a species lasts long enough to join the Overmind but not be remotely what the Overmind wants.