tbvfh the quality of discord rp is rather low across the board...i don't think the circles of people who write there vs. on forums overlap much. like if the server's not a.) adhering to outdated conventions _{(word counts, "literacy standards" that ultimately sum out to word count, passwords in the rules, etc.)} then it's b.) without any actual lore/plot structure past a single paragraph that's like "we are in a kingdom based off of [insert media here]." if somehow it's not any of that -- it almost certainly will be though -- then it's most definitely populated by 16 y/o's and/or using rl face claims.

like those jjk servers you've mentioned are probably just 102391923012903 rp channels nobody's ever going to write in for real + some rules and that's....it. they will most certainly die within the month, if not earlier.

+ many forums have been totally bombed from the internet, or (in jcink's case) the subscription's run out so nothing's accessible, therefore i don't think the data archival issue is unique to discord.

tl;dr: discord's not killing forum rp (imo). i really can't see the average forum rper wanting to write in the average discord server. forum rpers are just getting older and busier elsewhere, or tired of certain bs after encountering it for several years.