SHADOWBRINGERS

[BillyKaplan]

But then, if we follow your logic, should we pull Arenvald along to finish off every primal we beat? Or Fordola?

Arenvald is one of Hydaelyn's chosen, no difference if we had done it or him. As for Fordola, we'd need to learn more about her synthetic Echo before I can form an opinion. It might not even count for her.

Perhaps we should get Lolorito

While Lolorito is far from good, I wouldn't necessarily say he's pure evil either, and either way has no known ties to either Hydaelyn or Zodiark. The WoDs at this point in time have formed an alliance with the Ascians and therefore Zodiark, which is why I think they could even tip the balance in Darkness' favor.

If it were explained as "Every time Hydaelyn's chosen defeats a primal entity, they become more powerful," we'd have a genuine point here, but as far as we're told, that's not the case. We're still street level with immunity to lower level godhax.

You might've been able to claim that as fact until Stormblood. But in Stormblood we get cold, hard story telling us we got stronger over time because we saw the results as we kept fighting Zenos. At first we lost - then we fought Susano, and we managed to hold our ground against him in Yanxia. Then we beat Lakshmi, and we managed to beat him in Ala Mhigo. It was never clearly stated in-game, no, and there're other ways to explain our increase in power (my personal favorite is how we feed on people's faith in us like Primals do) but it's a very easy case to argue in favor of.

Why would this be an advantage if this weighted the world in Hydaelyn's favor, and why wouldn't we be running around doing what the WoD was doing (agitating beast tribes into summoning with the specific purpose of slaying that primal to gain Universal Points for their team)?

3.4 + 4.4. The Ascians are absolutely not in an advantageous position. This is a story-string that has been going strong since before the introduction of the WoD, and made clear once they showed up. We are an unstoppable force representing the Light. The only thing that even managed to slow us down was Zenos and we beat him. And now this voice from beyond - possibly the future - is warning us about a Calamity should we continue down this path of completely beating the Ascians.
Also, I'm not so sure if they agitated the Gnath into summoning Ravana. Their original reason for summoning him was waging war, I think that, and us taking Ravana down in the first place, gave them more than enough motivation to re-summon him. Which leaves this whole thing in an even bigger flux.

EDIT: addendum, random thought I had. Defeating the Primals in and of itself is not a victory for the Light, but us doing it is. Because Primals were defeated long before we showed up. We know, as you mentioned, the Company of Heroes defeated Titan. SMN questline talks about defeating Ifrit. In Limsa you hear about the last time they had to beat Leviathan. But all those Primals were beaten, yet still when the game starts it sounds like the Ascians are in full dominance, and the Scions have to wrestle the balance back tooth and nail and are still lagging behind.
Until you showed up. You, a Battle-ready Champion of Hydaelyn's. And that's when defeating the Primals started tipping the balance.