The reason The CW saved Supergirl in the first place is because of Berlanti. Black Lightning was passed on by Fox and saved by Berlanti's clout.
And keeping a show on budget is part of the job. I understand what happened regarding the tax incentives for Swamp Thing, but it's the job of the EP to keep the show running and that snafu is not the sort of thing that happens to megaproducers like Berlanti or Dick Wolf.
The reason the other Berlanti shows keep ending up on DCU/HBO/CW is because WB sees enough value in them not to let other companies take a shot at airing them in the first place. If Warner Bros thought Constantine or Krypton had a chance to succeed like Black Lightning and Supergirl have, they would have rescued those shows too.
Bottow line is every DC show Berlanti's name has been attached to is still on the air, one way or the other. Arrow is ending on its own terms, after 8 seasons and several highly successful spinoffs.