If aiki is essential to our aikido, then (a) we should be able to explain what it is, and (b) we should be able to find it on the mat without any extracurricular investigating. I think that if we have to look outside aikido to find it, then it was never a part of aikido in the first place.
It would be nice if that were so, but just like any other discipline, not every school and not every teacher is equal. And, aiki has been explained (here on AikiWeb). It's not a mysterious thing, only something that has been either kept a "secret" by some, incompletely learned and/or transmitted by others, or intentionally removed from a system in order to make it more easily taught and disseminated en masse.
If you lived in a country where baseball was introduced, but without the bat (let's say that the guy who brought it back from America decided the bat would be too difficult to manufacture), I suppose you would think that a bat was never part of baseball in the first place. And you probably would invent padded gloves for your hitting hand to protect it from 90 mph baseball impact, to make up for the absence of the bat (which was never a part of baseball...)