When I sat down with Jeff Kaplan to chat with him about Overwatch's beta (which feels like ages ago now) one of the first things I asked him is how his team intended on integrating solo content into the game. He made it clear that multiplayer was the core focus but over time, they'd continue to pepper in kernels of story and other modes: a practice they seem to want to continue.
Since launch we've gotten various events like a Halloween-themed horde mode, story-centric missions (Uprising and Retribution), and other goofy gametypes like a team of Meis versus a Yeti Winston and Blizzard is going to keep at it. Speaking to VG247, Geoff Goodman, lead hero designer, said that the feedback is clear: people want more stuff like that. Goodman says that the response has been "amazing" with high amounts of playtime to boot, noting that they're "pursuing more," to sate the "hunger" of fans to discover the underlying world of Overwatch.
While I do still enjoy Overwatch very much, I think the time has come to just integrate some of these missions into the main game instead of making us wait for yearly cadences. There's enough well-received meaty content here to put under a "campaign" banner on the main menu: and that might have already been implemented if it weren't for the fact that skins were so heavily tied to these events.
There's precedence there as Dota 2 introduced a campaign (that lives in arcade mode) though many other studios like Riot periodically wax and wane their events to similarly peddle limited-time skins.