1. The forums where one of the main reason why people would visit the website. To find opinions, theories, to debate.
Without the forums, Imdb will remain just a place to get information regarding movies, the ranks, directors and actors.
2. The feeling of the community will disappear.
Some users made friends there, had debates.
3. No more one place to read theories and debate movies. What will the users do now, search on Google for example: “Arrival theories”, look on reddit.
Go on YouTube and listen to some annoying vloggers?
4. No more place to read stuff about movies when going to bed. Me for example, I used to read from time to time, the imdb forums for some of my favorite movies like Alien, Aliens, The Shining, 2001 and other masterpieces. Since these movies had so much depth, people would go on the forums and still debate and post theories. No more place to do that. It was also very comforting to read about stuff that you like.
5. No more place to find answers to a movie. After wathing Arrival for example, a very complex movie, a user would just go and rank the movie but before that, read other people opinions on the forums.
6. Imdb could have just improved the forum boards to get rid of spam, bots and trolls. Just use a system like reddit with upvotes and downvotes.
YouTube has comments and there is a lot of bad things written there but YouTube didn’t just disabled comments.
7. Sadly, Imdb forum boards was a place of comfort for movie fans. They will get less traffic, less exposure.
Sad, hopefully they will reconsider.