I used "probably" as a figure of speech. The source of my claim is found within Oribos, the Arbiter, and the Pantheon of Death, and how their entire mission and existence is to safeguard the Shadowlands, and the design of the First Ones. Hell, it's even stated that if the Arbiter falls, everything the First Ones had designed would've been for naught.
The Titans, the Void Lords, etc are that exact same way, but for their own cosmic powers and respective realms.
"I feel like if the First Ones "crafted the cosmos" then they left the universe to its own devices. But until more is revealed, who really knows." They cannot be true, since they made the Arbiter and other factors to keep the Cosmos in check, and the other Pantheons came to be to keep the First Ones' design in balance, therefore being "extensions" of theirs, since they're continuing what the First Ones made. Nothing implies they just left it or anything. They're probably fucking around us right now or some shit. We don't know.
Also, watch the fucking interview OP linked. Steve talks about how the First Ones are the level above the Cosmic Pantheons. Hell, in other interviews, it's stated that the First Ones are linked with Cosmic Pantheons such as the Titans and Eternal Ones. Only further backs my claim.
And since you have no proof of your disagreements outside of personal feelings, and since I actually have proof of my claims, here you go:
https://www.wowhead.com/news=316807/...ser-elune-sylv
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So I don't want to spoil too much right now, but I will say that they are tied into the other Pantheons we have seen before like the Titans, and going through the Shadowlands, you will meet the Pantheon of Death, which are the leaders of these Covenants: The Winter Queen, The Archon, those kinds of figures, and they certainly have knowledge on the First Ones, but what they have to share… we’ll just have to wait and see.
To say the Cosmic Pantheons are extensions of their all powerful authority is not at all a reach, and while it is a speculation, if is pretty backed up, even if the First Ones are a little iffy. Especially since Steve confirmed that: A. The First Ones are connected/tied with the other Cosmic Pantheons, and B. They're the level above the Cosmic Pantheons.