This summer (late August) I flew Delta from TPE to SEA in economy because Delta was cheaper than Taiwanese airlines by more than $100. The economy cabin was less than 2/3 full. I had a peek into J cabin, it was almost empty. It's probably just one off and all other dates are fully occupied in J?!
To be honest, if Delta is pricing its premium cabins at or close to JX, BR, CI price, why would anyone fly Delta (other than they are "locked-in" to Delta's loyalty program)? I know I wouldn't. Taiwanese carrier are so much better than U.S. carriers in premium cabins, better service, better food, better amenities, better and cleaner toilet, better and cleaner seats, better looking crew (yes I prefer better looking crew). And there is no Delta One lounge at SEA anyway.