Actually, Discord channels will keep their entire history... it's just a pain in the neck to read them compared to a forum. (Of course, a suitably old and well-used forum can be a pain in the neck to dig through as well, but we're a long, long way off from that point.)
Okay, call me ignorant. How easily searchable is it? How organized is it? Because, from what I've seen, there's channels, and each channel is like one giant "thread", and it's hard to follow a conversation when it's not partitioned out into topic-specific threads.
Searchable, yes, if you can remember an exact phrase or who started the line of conversation you want to read. Organized, not at all, for exactly the reason you give.