I think that in 5 or more years forums aren't going to worth that much
I do agree with that. Forums boomed around the beginning of the 2000's, then I'd say started to decline in the early 2010's. The rise of Facebook, Twitter, Reddit and all of the other Social Media's out there is what's killing forums. Nobody want's to find a forum just for 1 certain topic. Everybody wants to talk about anything.
I have started a General Discussion before(But I kept shutting it down) and if I were to start one again, I'd make sure I let user's talk about as much as they want. That's what General Discussion forums are for, right?
But yeah. Forums won't be worth as much in the next 10 years. Unless we start a revolution or something, I dunno.
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But back to the thread's question, I'm not sure what I'd do if I were to shut down my forum. I'd probably do what everybody's doing. Send emails, write an announcement or sell it to some other person. I'd probably host a 14 days event which is essentially the countdown timer until the forum closes. I'd cut staff off, one by one. I'd also start removing features as they won't be useful anymore seeing as the forum is closing.