The original idea of the possessing alien was ridiculous. How can a human-sized being exist within another human-sized body? It's an anatomical absurdity, unless it skins the human host and just wears the skin around itself. The Goa'uld as fairly small parasites made more medical sense, as well as providing a good explanation for what evolutionary advantage these creatures would gain from the ability to take over larger, more dexterous hosts.
Any sufficiently advanced technology would seem like magic to the less advanced. A human sized parasite possessing a human sized host is no less ridiculous than a TARDIS being bigger on the inside, and how many frigging years has Doctor Who been on the air?
The goa'uld as fairly small parasites is just more of the same for the producers who moved from Outer Limits to SG-1. There's nothing brilliant or innovative about them.
And an eel being smart enough to completely take over the operation of a human mind and body makes no biological sense.