Where is the technology to do that (decentralized forum)?
it's not impossible to make something like this, as someone already created decentralized twitter alternative called nostr.
If Nostr is possible than forum should be even easier to create, but I am not sure how this would be moderated in real life and if people would even transition to new forum.
Would you run a node for free?
Not everyone needs to run a node for this, and with pruned nodes it's even easier.
We have hundreds, thousands of forums in the internet. This is among the best of them. I don't think there is much to change here.
Nothing can survive without any changes and improvements, including bitcointalk forum.