You are using colloquial "theory" as a guess, and that casual use doesn't apply to scientific studies. *Theories" are proven.

Your guess is a random and clearly biased hope that a study will eventually be done to demonstrate that sexual gratification of the genitals is interchangeable with sexual gratification of the feet.

Ask yourself a few pretend "control" questions. Can you orgasm from stimulation of your feet?

Can everyone orgasm from a scat or pre fetish? Can most people orgasm from furry time? Diaper play? Pain, humiliation?

We've all been exposed to social, familial, and situational stimulus which shapes our preferences.

People born in the 60s who experienced their sexual awakening in 70s skin mags are more likely to enjoy natural grooming, while humans who experienced puberty in the 00s are more likely to expect and enjoy hairless genitalia.

This isn't difficult, and it doesn't need to be. You have certain preferences based on a combo of biology and social stimulus during your formative years, and the best you can hope for is to stumble across a partner who shared your experience.

But you can demonstrate by either science or medicine that a foot fetish is "normal" because brain imaging any more than you can demonstrate that a shit fetish or a puppy strangling is normal because brain imagining.

Your interests deviate from the norm, but that isn't a condemnation of your proclivities unless you are hopelessly unable to reach orgasm by any other means.

If so, you're pretty much limited to other fetishists who match your kink or sentenced to a term of CBT to help you learn to enjoy sex with typically developing people.