Old-school corporation-run internet forums being killed off in the 2010s was usually due to costs of moderation (IMDB, Amazon) or brand-protection reasons (Lonely Planet) or both.

At the time, I was angry at this as a user. But looking back, the forums would have declined anyway. With most people today using the internet from phones, and younger generations chiefly interested in video, there just isn't the same societal support for thoughtful long-form text typed from a computer keyboard. The surviving independent forums I'm on are greying out at best, or have shrunk to a few cantankerous or dotty people at worse.