The CW’s Arrow-verse shows, which started off in 2012 with Arrow, currently dominate that network’s ten hours a week that it programs, but they are definitely in state of ratings decline. Arrow is in its third year of falling ratings, The Flash has slowed by 17% from last year, Supergirl is down by 24%, and Legends of Tomorrow is down by 14%. Black Lightning, which has not established a connection to the Arrow-verse yet, had a decent debut in January, but does not look like a savior of the superhero shows at this point.
All that said, none of these shows are in danger of getting cancelled or ending this season because The CW–aka the Happy Net–likes to renew shows, not cancel them. And their Arrow-verse entries (including the tangential Black Lightning) are still the best performers on the network along with mainstay Supernatural. So I expect that all five of the superhero shows will be back for the 2018-19 season, but I also believe that the declining ratings and franchise fatigue will bring changes to the Arrow-verse possibly as soon as next year. There are already strong indications of that by the direction the network is taking with upcoming development. It currently has quite a number of shows it is working on (you can see a rundown of those at this link), and none of those are superhero related. With only ten hours a week available for programming, the network has to do some shuffling to free up space for new shows. But which will be the first of the current superhero crop to go?
Arrow and The Flash currently have or will have by season end over 88 episodes (four full seasons for a broadcast net show) which is the number the syndication market prefers to see for an extended encore run. Supergirl will get to that mark by the end of next season, one of the reasons that one is a lock for renewal. Legends of Tomorrow is much further away (51 episodes by the end of this season) because it has had shorter episode orders, but that does not put it in danger of cancellation.
So among these four, I believe that the first to be impacted by impending changes will be Arrow. It is currently the lowest rated of the network’s superhero shows and it already has enough episodes for an extended syndication run (138 by season end). I would not be surprised if that show were to end up getting pushed to a later start next season, a move that has become common for The CW’s flagging franchises, and if it only receives a thirteen-episode order. It is also possible the network could also announce next season as its last, though with plans of rolling characters from Arrow into another show, most likely Legends of Tomorrow.
And then after next season, it’s possible that The CW could cut back to only two Arrow-verse shows (not counting Black Lightning, see below). Supergirl will have made it past the syndication stretch by that point and perhaps the network will decide to keep just The Flash and Legends of Tomorrow on the schedule. The former is the top-rated Arrow-verse show and the latter is the team-up show that can accommodate characters from both Supergirl and Arrow. I had previously thought the network might try to do a Brave and the Bold type team-up show that they could run any of a number of characters through, but now it looks like The CW is not interested in adding another superhero show. Another option would be to keep just The Flash and Arrow going (sticking with the best-rated and the original show) and roll characters from Supergirl and LoT into both of those (I consider that a possibility because Stephen Amell may be under an extended contract).
Most likely scenario (not taking actors’ contracts into account) is that Arrow has no more than two more seasons after this one and the same for Supergirl. Legends of Tomorrow could stick around longer to be used as a catch-all for characters whose shows have ended (or flip that with Arrow). And I believe that The Flash has several more seasons ahead of it (assuming The CW keeps its doors open that long) and could also pick up orphaned characters in regular or recurring roles. But the Arrow-verse is definitely not the powerhouse it once was and its network is certainly in the market for the next show or shows that could take over as its flagship series.
As for Black Lightning, it’s future is up in the air right now. A second season is pretty much guaranteed unless its ratings tank in the coming months. After that, it will all depend on how well the show holds its audience, both with linear viewing and its encore runs on Netflix. And definitely consider the possibility that this one could be pulled into the Arrow-verse proper at some point, similar to what happened with Supergirl. If Black Lightning’s viewership drops off too much, the show could face the ax and the main characters could get relegated to cross-over and/or recurring roles on whatever Arrow-verse shows are still on the air.
The franchise definitely has had a good run and still has a few more years ahead of it. But dedicating four or five (or even three) hours of Prime Time real estate will become less and less viable as the audience for the shows continues to dwindle. The Arrow-verse will not go away completely in a year or two, but I do believe that it will contract to fewer shows within a couple of seasons.