I don't think anything has ever really been a suitable replacement for forums. Just bring back proboards and PHPBB, with an automatic CC-BY or similar license, and some archiving and exporting tools, and maybe a mobile app.
https://discourse.org/ is the closest thing I’ve seen to what you’re asking.
I'm actually looking for something like this. Discourse pricing seems to be around $100 per month minimum for a group of 500? It's a huge leap from the forums where minimum cost is about $8/month.
Discourse is free software [0], you can simply host it elsewhere, the pricing you're looking at is their managed option, which while convenient absolutely isn't necessary.