Ultimately the value of forums has always been, and will probably continue to be, that it is an archived and searchable database of particular hobbies/communities.
Personally, sufu in its glory days was that for:
marketplace for specific designers/brands
Instagram and other photo-sharing things basically killed sufu's value as a "real-time" feed for waywt-type photos. However the archival aspect of sufu is still unique. I can't imagine trying to find a specific photo from a user on IG since it'd be a manual exercise in scrolling through an infinite feed. But if photos/posts here are tagged/captioned well, all it takes is a quick search and I can refer back to something years ago. Impossible with IG, etc.
Grailed largely took away the marketplace aspect, but users still want to avoid fees so sufu still has room to reclaim this area.
Fashion discussion is where sufu really beats out the competition, specifically because it's all archived. I can see posts from ten years ago, and if I were super committed and interested in a certain brand/aspect I could literally go through a large part of its history right here. Reddit really took away from forums because instead of having to bookmark/go to 10 different forums for different hobbies, it consolidated several interests into one site by way of subreddits, so now I just need to remember one site. But what Reddit gains in convenience it loses in archival discussion. Threads there have a lifetime of a couple days (talking about niche subreddits, not the FP)...if you're not around for the first few days when a thread is started and instead you find it weeks, months, years later and you ask a question or want to continue that conversation you've no chance. It's like you found out about a party too late and when you show up everyone's had their fun and now they're all gone--there's no one to talk to anymore.
But sufu (and all forums, really) are an on-going, and in a lot of cases, never-ending discussion. If someone replies to a thread that's months old it generally gets bumped to the top of the list and users are reminded and can pick up right where it left off.
So how to capitalize on those strengths? I'm not sure. For photos maybe make it a requirement to include relevant tags/brands/captions? Improve the marketplace so it's easier to upload photos and make it mobile-friendly. Make the search functionality throughout the site great so the archival aspect (the one true unique selling proposition, IMO) can continue. In its current form it kinda sucks. And maybe our mentality to new entrants to a discussion/thread that ask an already-answered question has to change too. That is, I'm sure we've all experienced a first-time poster coming into a 5+ year old thread asking a question that's been asked hundreds of times before and we tell em to "just use the search function." While that is a valid answer, the effect of a newcomer asking a question bumps a thread to the top, bringing more eyes to it and with it the potential to continue a discussion that might've tapered off awhile ago. Plus answering their question straight away would probably be a positive experience for them, giving them incentive to return; whereas shooing them away to the search function might leave a bad taste in their mouth and they'll never return.