There are plenty of references in ancient and medieval primary sources to a morning meal--references in a way that very much suggest such a thing is standard in the culture at the time, at least for the class of people being described (or whom the people reading the texts would ascribe to). From ancient Sumer:

When I got up in the morning, I looked at my mother and said to her, ‘Mother, give me my breakfast; I want to go to school!’ Then my mother gave me two loaves out of the oven and I satisfied my hunger before her eyes. Then my mother gave me two more loaves to take with me and I went to school.

(in Römer's Texte aus der Umwelt des Alten Testaments, here ripped shamelessly from Women in the Ancient Near East

and Polybius, describing the Second Punic War:

For directly it was light he sent his aides with orders to the tribunes and men to arm, as soon as they had got their breakfasts, and parade outside the camp. The order was obeyed with alacrity because the men suspected what was going to take place...The sudden approach of the cavalry to their camp, and the simultaneous appearance of the rest of the army getting into order, left the Carthaginians barely time to get under arms. Hasdrubal was therefore obliged, without waiting for the men to get breakfast, or making any preparations, to despatch his cavalry and light-armed troops at once against the enemy's cavalry on the plain.

Polybius, Histories 11.22

The point being not that This Is What Happened, in either case. But rather, that the author doesn't seem to think the "breakfast" part of the equation is special, and more to the point, doesn't expect their readers to think as such.

And still so very much before Kellogg, John Melish traveling through U.S.America at the beginning of the 19th century talks about breakfast all the dang time. (Just search for "breakfast.") Highlights include a discussion of proper eating:exercise balance while at sea, staying with a family whose members were are DIRT sick but still managed to turn out a breakfast for him, and him insisting that really, a simple breakfast was fine, but the hostess insisting on setting out quite the spread.

Now. Have there been changes to breakfast in recent times? Yes, although it's really strange to see Kellogg brought up in this context.

U.S.Americans are kind of famous for, in the west, inaugurating the massive gluttonous breakfast as a regular course of action. On one hand, the "lavish breakfast" idea came out of the slaveholding South for the obvious reason. On the other, even northern Americans got into their own act. Colonizing and antebellum Americans transformed anything vaguely bread-like into "bread" suitable for breakfast--most especially, doughnuts and waffles.

But Kellogg, as I've talked about earlier on AH concerning vegetarianism, came out of the spa/sanitarium tradition, which is to say--health food. His original experiments with food went towards developing alternatives for meat--especially working with nuts.

His breakfast innovation--cereal--is crucial first of all for being ready to eat. No cooking a la oatmeal or cream of wheat required. Part of his idea was also that it be relatively soft, to help out people with no teeth or tooth problems.

And in conjunction with his overall mission of a health food, both John Harvey and brother Will pushed to adapt their creals to a lower-fat, higher-fiber breakfast than what they saw as characteristic of "American" breakfasts at the time. (I'm not sure how class-based, especially with respect to immigrant populations, that actually was at the turn of the 20th century).

Did Kellogg help change the face of breakfast? Sure. Were he and his company fantastically influential? Actually, yes. But did he invent breakfast, even just looking at the NATO world? No.

Chalk this myth right up there with "paleo," and go eat a dang doughnut already.