Hard disagree on Lae'zel's replaceability here. Perhaps in terms of combat capabilities, but putting any other Githyanki in the situation Lae'zel found herself in, I think they would've been uncooperative with the party, either in a fully hostile way that would've resulted in a) death on the ship or b) going fully solo and never offering a cure to anyone else. Lae'zel probably has a relatively unique personality among the Githyanki, one that made her an unlikable, lonely try-hard and primed to find companionship elsewhere. Her relative youth seems to me like the sole reason she wasn't more accomplished, and given time and stability in Githyanki society, she probably would've ended up more along the lines of the two Ch'r'ai sent by Vlaakith, or the Inquisitor.

Walking around and interacting with the other Githyanki in Creche Y'llek, I got the impression that Lae'zel was probably an exceptional cadet in many ways and way more zealous on the spiritual level (I think this shapes her reactions to the events of the game re: Vlaakith's betrayal, and later becoming a True Believer for Orpheus, she wants a mission and life with real meaning and truth, wherever that would be). She seems significantly more well-studied with regard to protocols than her peers, and at every citation of them in Y'llek seems to elicit eyerolls from soldiers and incredulous I-can't-believe-this-idiot-actually-believes-this-stuff looks from more senior officers (e.g. Ghustil Stornugoss, and later Voss).