As someone who just did Korthia for the first time 3 days ago, to unlock SL flying, yes, Korthia looks like it was better than ZC.
Korthia has phases, Rift and normal, the artifact items to find and related research to unlock stuff there.
It also has some platforming challenges, which flight negates, and it unlocks organically, apparently.
The major flaw, I’d say, is that it’s too small.
And I don’t think the enemies or events are half as obnoxious as the ZC ones.
The Zaqali are a super PITA to solo as a mage, the events are all very hard or time consuming to solo as well, while I’ve seen none of that in Korthia so far, other than covenant-locked events.
I don’t know for sure because I’m overleved now, but I feel I’d like it better than the caverns and their terrible enemies.
The major plus for ZC is sniffenseeking and then snail races, maybe.
Let that sink in.
And Zereth Mortis is much better than Korthia, judging from the small amount of time I spent in there.
After the WQs and weeklies, I won’t be touching ZC, but I’ll be finishing Korthia and ZM.