In Build Content, Under Image, Alt Text should be a required field.
At first, I thought "What a Draconian approach!" and wondered why. I then remember my coworkers in our Division of Disabillty Resources and Educational Services worrying about their students and also protecting our campus from law suits I can agree with your suggestion to make that Alt Text field required. The person building the course can probably skate by it by typing in gibberish but then doing so will show that it was deliberate act.
So yes, I think this should be an easy change but I do not write the code.
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