Are these champs staying forever?

SUPER_PSYCHIC

Silver sable
Northstar
Negasonic TW
Arcade

Maratox

Yes every obtainable champ that they add to the game remains forever

Magrailothos

Why wouldn't they be staying?

They may not stay unchanged: they will go through the same rebalancing process as every other champion.

Lokx

We don’t know yet, they’re waiting to see if they get a better contract from dc studios.