The crossover isn't over yet, and there's no way that's the last of Routh's Superman, much less the end of Iris West, Black Lightning's family and Earth, or most of the other characters for that matter. If I had to guess, I'd bet the final episode is very much like issues #11/12 of the comics. They'll wake up on a merged Earth where Earth-1, Earth-38 and Earth-(whatever Earth Black Lightning is from) are all merged together...with Routh's Superman playing the role of Earth-2 Superman who was the odd man out and had no Earth to go back to.
I agree, they should be building Hoechlin's Superman up in anticipation of his new series, but he seems to be getting sidelined a lot in favor of Routh's version.
I'm loving what Routh is doing, but I'd like to see Hoechlin getting a bigger part in all of this.