Longer answer on why Forum has to go

Why does Forum have to be removed?

Forum was initially developed at a time when Wikia still built features exclusively on MediaWiki. So the Forum had to be built to work with that software, even though MediaWiki wasn't really meant to support a conversation feature of this kind. The developers did a good job -- many communities have since come to use and love Forums. However, making Forum work smoothly with MediaWiki was a lot like forcing a square peg into a round hole.

As a result of this suboptimal MediaWiki integration, the Forum has many issues, some obvious, some only noticeable to the most diligent and tech-savvy admins. For example: those "@comment" URLs showing up in weird places and sometimes never leading anywhere, or content that never gets removed from maintenance lists even after the issue is resolved. Some of the more glaring issues include long load times on threads with many replies, or that the layout of an entire page can break when one user forgets to close a  tag in their post. Forum's basic MediaWiki-based architecture was ultimately not set up to accommodate big alterations in the future and stand the test of time.

Over the years, we’ve kept Forum working well -- more or less -- by continually patching the cracks in the dam. This works for a while and most users don’t even notice (which is the point of the patches, to keep things together). But eventually you have to accept the fact that the dam needs to be replaced. Now that FANDOM has new options available for developing parts of the site, it has become obvious that trying to enhance a conversation feature despite the inherent limitations is not the right path forward. By rebuilding the feature from scratch instead of patching it up further, we can eliminate a lot of the problems mentioned and make sure Discussions won't have the same constraints and can more easily be evolved over time.