It takes optometrists 4 years to learn every procedure, test, and diagnosis related to vision and the eye (for the sake of space I'll avoid putting down a textbook size summary of these items) as well as have 1 to 2 years of hands-on clinical experience in a proctored setting. Even this is only the beginning and a good clinician with continue to learn and through experience grow into becoming a skilled competent eye doctor. Your question speaks about your ignorance of the complexity of the eye and visual system and although you prefaced your comment with a disclaimer, it certainly did come off as condescending and pretentious.
Optometrists prescribe pain meds, antibiotics, steroids, etc. and certainly do provide emergency eye care. They are primary eye care providers and in rural settings are often the only trained eye care professional in the surrounding region. Optometrists do perform minor surgeries in nearly every state (such as foreign body removals) and are quickly adopting laws to perform laser surgeries and cosmetic lid procedures as well. Health care laws are changing rapidly and dentists are certainly realizing this with dental hygienists and dental therapists in some states now setting up independent practices to perform oral exams and perform most of the tasks that had previously required dentists, including extractions, fillings, cleanings, dentures, etc..
OMDs do not receive the same training as an optometrist. Their knowledge of optics, refraction, and binocular vision is generally shallow. In some instances they can perform similarly to an OD but as you know, an OMD requires 11-13 years of training while Optometry requires 8 years (including undergrad) so I don't see OMDs as a replacement for ODs.
Techs are trained in performing tasks but do not have a doctors in-depth knowledge of the underlying mechanisms or the potential problems and diagnosis. Techs are techs. They aren't doctors. I would note that my last visit to the dentist, I spent 95% of my time with the dental technicians. If I were ignorant of the training that a dentist requires, I would say that it seems like 95% of a dentist's job could be performed by a dental tech.
Optometry schools often reteach subjects from undergrad such as biochemistry, anatomy, microbiology, and physiology but this is no different from medical school. Dental schools reteach biology, anatomy, biochemistry, and microbiology as well.