I'm very happy you brought up the Egregore. I'm fascinated with the concept, and would love to find a group I can trust that, one day, might be able to make one. I think Egregores could be useful for research purposes and other means if properly created.
To your actual point, I think there's little to distinguish an "ancient entity" from a newer Egregore. In fact, the time only means they have continued to exist. If the time has continued to ramp up the worship they receive, to the point that they are unstable and would fail without it, there could be a problem. If time has seen them run low and become a starving "ghost" (i.e. Preta, or preta-like), I've seen what that does. But yes, if they worked smart and used that energy to build up their power correctly over centuries or millennia, they may not need constant adulation. Though they will definitely crave it, from what I've seen.
Imagine if you will, a form of love, euphoria, and release that unmakes you. After experiencing a hit of this, just a taste, you are gone. You have no concept of right and wrong, no concept of past or future, no concept of active or inactive, waking or sleeping, pain or pleasure. All you know is that you have experienced something truly transcendent. Now you only feel like you're alive, like your world makes sense, while you are experiencing that all-surpassing harmony. I know, hard to really picture. But when a throng of humans treat you as superior, inherently, for generations...and supply you with sacrifices, praise, and the fates of their children, it is that kind of soul-shattering vibration to which you become addicted.
Sounds from what you wrote like you've met "sober" entities. Beings who have achieved this scale without losing themselves to a truly thunderous wavelength and frequency. Or who have recovered. Not beings to be taken lightly, for sure. If they are capable of that, I'd say whatever word they want spread, or deeds they want done, will happen whether you help them or not. Consciously, at least.