Someone on TFN posted this in response to B-Wings appearing in Rebels:
"I would just take this as another example of Filoni's arrogance, thinking it's okay to continue to stomp on Legends lore so he can put his own stamp on things. A Filoni-lover may excuse this by claiming ignorance, but that whole team claims to love Star Wars way too much NOT to know what they're doing when they (intentionally) violate existing/Legends canon. And yes, I get that Legends no longer has legit status, but I see no reason to slap longtime fans in the face when you can use a different or new ship/character/planet/weapon instead of changing existing material (see Depa Billaba). 
"
Why in the world does the date of a ship's debut matter so much?