Didn't say that previous FF iterations don't have influence, I'm just saying conceptually in the world of FF THF and NIN function differently even if they have similarities between one another. And though NIN is treated as an advanced job to THF in some FF iterations such as Tactics and FF1 not all iterations of jobs in previous FF titles have maintained the same functions and roles throughout the series nor the same specialties, and sometimes when specialties did cross over in previous titles it may have been more out of lack of roles or necessity.
For example lets take SCH, they're renowned for being users of black and white magic in previous FF titles but was never a pet/summoner class which is contrary to FFXIV's SCH which is more closely related to FFIII's Sage, that is an example of role change. For an example of lack of roles/necessity you can look at Rydia, she's primarily classified as a Summoner but she's also a solid Nuker and for the early parts of the game she's your only real healer between temp party members like Porom, Palom, and Tellah.
Edge as you brought up does share characteristics from NIN and THF, but there isn't a THF oriented character in FF4 either, so I can see Square being a little lax with roles in that instance, but it's not like you see similarities between Shadow and Locke conflicting with one another in FF6 or the job system in FF3 and FF5 where both jobs are present.
BTW Yuffie is a Thief and Ninja by choice of her personality, and technically anyone in FF2, FF7, and FF8 can theoretically be a THF/NIN because there isn't a set job to any character.
Don't mistake honor with nobility. I mean sure you get the typical rogue ninja, but many ninja clans societies still functioned with the caste system of Feudal Japan at the time, and didn't typically make a move without the consent of the head of the household/clan/estate/whatever. The family honor and loyalty still meant much to them weather or not the family or the individuals intentions were of noble origins.
Thieves on the other hand typically act independently of one another, and though guilds have historically been established to maintain order just like any organized crime syndicate, a typical thief's loyalty can more likely change on a whim or be bought out than compared to a Ninjas.