e621 is an art aggregate site and not necessarily a place where artists can maintain a gallery so to speak.
As far as what will "replace" FurAffinity, there are a number of players out there right now. Inkbunny, as previously mentioned, is a possible alternative however Orthodox Chrischanity is correct in noting that it has a reputation for "cub" art, so within the fandom people tend to avoid it unless they are specifically looking for that type of art. Inkbunny received a boost in traffic following FurAffinity's termination of all cub art a few years ago.
Weasyl is another alternative, and the most likely one that people will flock to. Weasyl is primarily an art site and artists have been trickling over there for quite some time, just not at the pace that the mass-exodus rate that we're seeing right now. My honest thoughts is that Weasyl will become "the new FA".
There are also some lesser known places that users may flock to, but most likely will not see a massive surge in new users. One such community is SoFurry which was established originally as YiffStar (The Yiffy Story Archive) in 2002 before shifting gears into a more art-centric site a few years back. SoFurry continues to attract mostly writers despite their reformation as a general art site.
FurAffinity had a good run, but I feel like they've been in the spotlight long enough. It's honestly only been a matter of time until they fell by the wayside, as is the circle of life online. First there was VCL, then Furnation, then FurAffinity, and now whichever community takes center stage following this fiasco.