As I understand it, a dragonbreak is meddling with time. A sort of retcon, only done in-universe.
The Selectives danced on the tower causing the middle dawn and Akatosh was born.
Only after that event, he had always already existed. Because a retcon affects the past as well as the point onwards from when it happens.
That it decidedly is not. While the Dragon Break has often been joked about as being the ultimate retcon device, this is not the case. There is no example in which the Dragon Break has been used to for retcon (which is not the same as extending a running story, mind you).
There have been four periods in the history of Tamriel during which time became uncertain. These are The Dawn Era, the Dragon Break (the one caused by the Selectives), The Apotheosis of the Tribunal and The Warp of the West. For all but the break caused by the Selectives we know that gods were walking mundus. Be it the Aedra, the Tribunal or Eight times the Brass God. From the Nu-Mantia Intercept we know that each god controls the ordering of events as they happen. The book "Warp in the West" provides several examples of what that might look like. Basically, when the gods take conflicting decisions about how events should occur in the world right now, it splits in two. One half follows the decision of one god, the other the decision of the other. When their control lapses both alternatives are merged back again into one. Note the similarity in confusion and impossible movements between as seen in the aftermath of the Warp in the West and the Dawn Era itself. As far as the Tribunal is concerned, the also Sermons note that Vivec had his feet cut off and replaced by feet made from Oblivion so to avoid harming the good earth. Implying that he gave up his control over time.
This paints a pattern. Gods walking Mundus, causes the Dragon break. With no other known mechanism to cause a Dragon Break we can only assume that during the Dragon Break caused by the Selective there were gods walking Mundus. And what other gods then the Selective itself? Their intent was to remove the Merish aspects of Akatosh, something they could not. Then what else to do but to replace the divines with pure-blooded-human gods? What else to do but assume control over time, lest that Altmeri lizard does it?
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I suppose I should also make the argument that a Dragon Break can not change the past in Tamriel. I can not make a conclusive argument here other then that it has never happen. The rather famous example, Vivec having always been a god does not make sense. Had this been true, had the past indeed been changed we would not know about a mortal Vivec.