Interestingly enough, some of these "disciples" of the founder have stopped using the esoteric notion of "ki" while others have embraced it. It's certainly not universal that people believe it's a central element in aikido.
Also, I'll add that the founder was hardly the first to use the term "aiki." That term has been around for hundreds of years in a martial context and has been used in arts like Daito-ryu that do not speak about "ki." If you can read Japanese, a Daito-ryu practitioner by the name of Kimura has written a book on his teacher Sagawa sensei who supposedly used this "aiki" very effectively -- people would fly off of him as though flung at just a touch.
The way I personally approach aikido is primarily through the physical. The spiritual aspect comes into play from training to refine my skill in the art. Of course, some of the concepts like relaxation and such come into play in a non-martial arena, but I can probably get to these same demons by doing another non-martial activity like sitting.