Of course, the Arrow-verse idea makers could find another way to put the worlds in jeopardy, although changing the story so drastically might upset the most loyal DC Comics fans who tune in.
An adaptation of “Crisis On Infinite Earths” would be very hard to pull off without this character.
That said, we have yet to learn if “Elseworlds” or any future event will introduce Anti-Monitor, so it’s possible other plans could be in the works if “Crisis” is happening.
Without Monitor, there’s no Anti-Monitor, and thus, no threat to the Multiverse.