You're just making excuses and exceptions for 13's situation. This is what you said:
Not 'huge decline in console JRPG game sales'. You also use Bravely Second as an example when I can just as arbitrarily exclude it for being a handheld game like you exclude Xillia for being a smaller seller, despite Xillia being the better comparison to 13. (Not to mention you used it for a comparison yourself.) Making excuses is handwaving away data that shows that 13 and Lightning performed significantly worse with each iteration through faults of its own.
13's sales declined very sharply. If you want to argue that there's a steep drop in console game sales, then you have to contend with all these other titles that managed to grow significantly. Resident Evil 5 sold quite a lot, and RE6 managed to achieve 91% of its predecessor's sales. How come 13-2 and LR couldn't do that? Simple, instead of maintaining quality and orienting towards wider appeal, 13-2 was a rush job reusing most of 13's assets and LR turned internally towards the Japanese otaku market.