Although this is a conjecture, Venat specifically states that she must be the one to become The Heart of Hydaelyn, I assume, if my theory is correct, it is to make sure Hydaelyn doesn't have the desire to siphon aether like we see other primals. And possibly not temper people either.
Why exactly would Venat specifically mantling Hydaelyn prevent that?

I say this because what we know states that Convocation didn't see that summoning Zodiark would only delay the end days instead of adverting them.
We have no idea why the Anyder posit this.

but the process of changing the laws of reality to stop the final days sets off the process again to restart the final days.
We have nothing at all stating this, that what Zodiark did would somehow cause the Final Days again, based on his actions.

No evidence is given that Zodiark would function differently that any other normal primal, other than he is an elder primal so his scope of power is just well beyond normal primals.
No evidence is given that Hydaelyn functions any differently from Zodiark. The idea that she doesn't maintain herself through consuming aether comes about purely due to the fact that she's been getting weaker over time, which is only a contradiction on it's face and ignores that we have instances of her consuming aether to empower herself anyway.

This is how I came to believe that aether consumption is the key triggering the final days. And to continue the logic, it had to be creation magic and the abuse of aether caused by casting it that leads to the final days in the first place. And, until patch 5.5 gives us a better explanation, the towers Fandaniel placed are possibly siphoning planet aether to Zodiark on the moon.
So aether consumption was the cause of the Final Days, which the planet sought to rectify by eliminating huge quantities of aether by leaving the world a barren wasteland, which the Convocation halted by sacrificing huge amounts of aether, which they also contributed to by sacrificing huge amounts of aether, which was solved by Hydaelyn consuming a huge amount of aether to be summoned, which has somehow never reappeared again despite entire Umbral Eras being caused by the mass siphoning of aether from the land?

And this is all ignoring that the Ancients used their own personal aether for creation magic to begin with, not the planet's aether. Also, a better explanation for the towers already exists, namely that instead of them beaming aether from the planet to Zodiark, they're beaming aether from Zodiark to the planet, hence the tempering (I don't think this is exactly correct, but it's more mechanically sound based on what we've seen them do than the opposite).