I've asked myself many of these questions, mostly around why do I continue to run the community/forum I am running.
We have a niche, it's personally interesting to me, we have subject matter experts and have content that people care about.
Why do forums fail? Thats easy. Facebook
I think that Big Social is a poor substitute for fourm type things.
and some users with 0 posts
I was just looking at setting up notice pop-ups for valid users registered as far back as 2000 (board was migrated to XF this year from phpBB) that have never made a post but visited since 1/1/2020... there were more than several. Damn lurkers. I found that really interesting and would like to know more about that motivation (or lack of). Also realized that Notices system does not have a full filter set like Search for Users i.e. search for registered between and made 0 posts (feature request).
great core group of users more than willing to pay their part of the costs associated with running the website
This is important depending on your goals. I'm not necessarily in it for the money (but would be great if it was there!). I tend to think that my members have disposable income (old cars forum)...
Broadly speaking, there is little or no room now for general chat type forums. To succeed you need to specialise in a narrow niche and become the world's most useful resource for than niche, or at least your country's best resource. If you can do that your forum will grow organically.
Agree on that.
So, would I start this forum/community from the ground up starting today? Nope. Maybe a facebook group/page... It might attract more and younger users but it would not be anywhere near a resource as it is as a forum. I've been lucky since it was a community in-place since pre-2000 and had a decent membership and I was in the right place at the right time and it changed hands etc - For me, my goal is to preserve the knowledge base contained therein. The lost-art knowledge these guy share is amazing.
In my mind - a 'real' forum is perfect for the community I am serving. If it wasn't XF, it would have been phpBB. A wiki could work, but it's just not the same experience.
My users have commented re: XF on ease of uploading media, albums, thread options/types. They're happy. There are competing sites that have moved from XYZ forum to a FB Groups style format. I am 'going after' those users. When they arrive, they say they're glad they found us. We have great long time moderators (sadly, one passed from C19 in November - found out January...)
Just my .02.