What's the point with that quite lackluster story in Endwalker?

[Silver-Strider]

Correction.

An exhausted dragon who just got the other half of her missing power back and was transformed into a really powerful Voidsent.

That was subsequently beaten and exhausted after our fight with Golbez. Not like we haven't found Dragon enemies before with Nidhogg and Shinryu and even if they had 1/13 of Zodiarcs power infused into them, we already beat at 7/13 fused version of Zodiarc and I doubt a single Dragon of the 1st Brood would be able to make up the power gap between 1/13 and 7/13.

[RyuDragnier]

That was subsequently beaten and exhausted after our fight with Golbez. Not like we haven't found Dragon enemies before with Nidhogg and Shinryu and even if they had 1/13 of Zodiarcs power infused into them, we already beat at 7/13 fused version of Zodiarc and I doubt a single Dragon of the 1st Brood would be able to make up the power gap between 1/13 and 7/13.

We don't know how many Voidsent Golbez has been feeding Zeromus, if any. Keep in mind that Dragon Eyes are ridiculously powerful, as we saw with Nidhogg (where we needed our own eye from Hraesvelgr to be on equal ground) and Shinryu (having been a primal made of hatred AND the eyes).

[Xirean]

That was subsequently beaten and exhausted after our fight with Golbez. Not like we haven't found Dragon enemies before with Nidhogg and Shinryu and even if they had 1/13 of Zodiarcs power infused into them, we already beat at 7/13 fused version of Zodiarc and I doubt a single Dragon of the 1st Brood would be able to make up the power gap between 1/13 and 7/13.

It's not just a matter of power scaling but also location. We literally cannot kill it in the 13th because it won't stay dead. We've also never seen how a being of that power is effected by the rebirth nature of the 13th either. It could be more or less unstable so we really don't know what will or won't happen to whatever entities are inside in, notably the dragon. Reasons why Y'shtola is plotting to fight it on another shard where we can both kill it and better control for what happens to the dragon afterwards.

Edit: Also the warding scales will likely give out if it's fought in the 13th. Hard to fight both the area itself and the primal voidsent.