I think we can rule out that case, the CW is so much lower ratings wise than CBS they’d probably be more inclined to axe it before the opportunity arose. There’s a few ways it could happen, encouraging CBS to do the crossover is the best way round it really, because the alternatives are doing the crossover anyway in spite of what the networks want which may cause some upset among the execs (as happened in the first attempted Arrow/Constantine crossover which never went into production) or waiting until one of the networks lose all their shows (because CBS may very well start demanding Supergirl universe spin-offs) and then dragging them on board.

And I think that will probably be the issue for a little while, provided that “Supergirl” ratings don’t fall to a point where CBS thinks that it could help them.

The only real issue, is the one that’s been given officially, and that’s CBS being old fashioned and not wanting to cooperate with other networks.