Narnia is an extremely Christian children's series. The Last Battle's imagery borders on Islamophobic in places. The kitchen-sink world building doesn't hold up well to an adult reader, and the relentlessly preachy Christianity even less well. Unless perhaps the reader happens to be a bishop.

There were dozens of stories set during this period with a heroic Luke. Disney decided to rebrand them all as "Legends" so that they could develop a second, clashing continuity.

Wouldn't Darkseid just boom-tube out?

Any Batman villain would be screwed though. This is why for the most part real superheroes stay out of Gotham. Not just Supes but usually the Flashes, Green Lanterns, etc who could clean the city up permanently in an hour or less, leaving Batman useless. That's the problem with a shared world with heroes of such disparate power levels.