I don’t think anyone new is going to get walruses for a while. The current population is dependent on wild rescues to maintain numbers as I understand it, and if the aquariums want to get breeding up they’ll probably need to increase the number of individuals at the existing holders to better mimic wild social groupings. At least, that’s the theory.
It also just doesn’t really make business sense to keep a large exhibit holding a redundant species just in case there’s a random unreleasable elephant seal. I’d like to think if there is one they could probably hang out at Alaska Sealife Center or some other more rehab-focused facility for long enough to ship the sea lions or penguins / pelicans out.