There are a few points you missed.

1./ at this point resummoning Hydaelyn would be difficult as only the Sharlayans and the Scions know of her actual form, most of the population who knows of and believes in Hydaelyn think of her as the Mothercrystal.

2./ Zodiark is also not a widely known summon. The Ascians, the scions, and maybe the Sharlayans. It’s not as if he is common knowledge. Most would go to the twelve if they wanted to summon someone or something new.

3./ As previously mentioned warding scales exist now, and so long as we have those tempering is not an issue. And porxies exist in case someone does get tempered.

So the likeliness of facing Hydaelyn or Zodiark again is in my opinion minimal outside of maybe an Ultimate.