Is he, though? I don't recall the narrative giving us any actual indication he retained the vast repositories of knowledge that an Ancient that lived through the Final Days of Yore and what came before might possess. I'm not saying that you're definitively wrong per se, but he always came off more to me as some sort of Sundered familiar of Hydaelyn's in a manner similar to Argos. He almost certainly wasn't the same man that he was during the era of the World Unsundered. Say what you will about Y'shtola, but rude and dismissive is certainly not one the adjectives I'd use to describe her. She's never actually talked down towards anyone, not in anyway that could really be considered dismissive. Most of the closest counterexamples are more snarky one-liners, like "Little Sun", and "Take you over my knee" and the more recent one when the WoL throws her under the bus beneath Thavnair's ocean.
I don't really think that a weak counterargument. So little of Y'shtola's actual upbringing and background is directly stated that such inferences based on what little context clues we do know are often necessary to piece together an accurate picture of what she's like. Personally, I'd argue that Y'shtola being somewhat emotionally detached upon discovering something new can actually be attributed to her apprenticeship under the Master Matoya. We've all seen what she's like, it's not unrealistic to think Y'shtola may have picked up some of her mannerisms in the process. It helps to tie the title of Master Matoya between the both of them as well. It's almost like the former is maternal towards the latter. Something to ruminate upon, at any rate.
Honestly I'd argue all the Scions could use some fleshed out backstories now that all the hubbub over the Apocalypse is out of the way. It'd certainly make them more tolerable, at least for me. How much do we definitively know about any individual member of the Scions of the Seventh Dawn? I empathize more with characters with fleshed out backstories and motives, and FFXIV ironically does better in this respect with it's antagonists and now all the major ones are dead and dusted. As an aside, this topic's title is kinda funny since Zero's story.... hasn't actually gone anywhere yet. While I can't really say I'm looking forward to where they take it, I function better when I give people the benefit of the doubt than when I condemn them based on a pre-crime. So, I dunno maybe the Void will be interesting. I know I have my own expectations for it, but it's simply not feasible to expect all those to play out as I desire them to.