What Does This Mean For Marvel?
Bendis' history with Marvel stretches back to 2000, when he and artist Mark Bagley launched Ultimate Spider-Man, which itself launched Earth-1610, aka the Ultimate Universe. Since then, he's written many comics for the publisher and played a key role in several of its most important events, including 2005's House of M, 2008's Secret Invasion and 2016's Civil War II. For better or worse, Bendis has undoubtedly helped shape the Marvel Universe into its current form.
Outside of line-wide events, Bendis has co-created some of the company's most popular and representationally progressive characters, but diversity has become something of a hot button issue for Marvel lately. In April, David Gabriel -- Marvel’s senior vice president of sales and marketing -- place the blame for flagging sales on diversity, and a retailer-only panel at New York Comic Con 2017 saw one direct market retailer expressing frustration with the company for changing existing characters so as to incorporate people of color and women. While there are certainly other creators and editors working hard to make Marvel more diverse, Bendis' departure may signal a return to classic versions of characters and decreased representation of people of color and women. However, Bendis has been the only writer to helm a solo book starring many of these diverse characters, so him leaving may also provide someone from a demographic the opportunity to bring a dimension to these characters that he couldn't bring.
A number of the characters Bendis has co-created are quite likely to continue being important past his departure. Jessica Jones, who debuted in 2001's Alias #1, has gone on to star in two Netflix series (with a third on the way) and has become one of the Marvel Universe's most popular characters. Bendis also created Miles Morales, who first took over the mantle of Spider-Man from the deceased Ultimate Peter Parker before making his way to the Marvel Prime Universe. The character will also appear in Insomniac Games' upcoming Spider-Man video game, with some hoping he'll be a playable character. Morales is also confirmed to exist in the Marvel Cinematic Universe due to the appearance of his uncle Aaron Davis (Donald Glover) in 2017's Spider-Man: Homecoming, so he may someday make a live-action film appearance in addition to starring in his own animated feature. As they're now important parts of Marvel's offerings across different mediums, it's unlikely that these characters will disappear when Bendis leaves for DC, though they might undergo some changes.
Of course, some of Bendis' creations might not survive his departure. Riri Williams, a young woman who first appeared in 2016's Invincible Iron Man #7, has taken Tony Stark's place for the duration of his comatose state following Civil War II. The current arc of Invincible Iron Man teases the return of Tony Stark. Bendis tried establishing Riri's continuing importance to the Marvel Universe in Generations: Iron Heart & Iron Man, so there's a very good chance that she will continue her superheroics despite Tony's return. However, she could also gradually disappear going forward, as she's not yet as culturally entrenched as Morales or Jones.
It may not just be comics that are affected by Bendis' departure; Bendis was part of the now disbanded Marvel Creative Committee, which oversaw the adaptation of Marvel properties from script to screen. He's consistently worked as a producer on the company's shows, and he was involved in the pre-production process for a currently untitled animated movie starring Miles Morales. Although Bendis will still get credit for his work, he may not have anything to do with these projects going forward.