Lions will most certainly be at Melbourne long term. They’re a staple of Melbourne and have been for decades (going back to the Lion Park days). They’re hugely popular with the public and they are one of the main draws of the current Carnivores precinct.
The enclosure is relatively small, but is suitable for a small pair of males. Going down the line, say 20 or so years, I can definitely see Melbourne reconsidering holding Lions. Spotted Hyenas seem like the only feasible replacement but if it is in 20 years time when they lose them, I’d imagine the whole precinct would get a makeover.
I think the elephants leaving will be a game-changer. 20 years ago, I doubt anybody could imagine Melbourne Zoo without elephants. They’ve been there since 1883! After they leave, international tourists will still come to Melbourne Zoo in the droves and see signage encouraging them to visit the elephant herd at Werribee.
The elephants were phased out for space/ethical reasons; but the process will no less demonstrate Melbourne Zoo can continue without them, just like the show has gone on without bears. I see no reason visitors can’t enjoy seeing Sumatran tigers, Snow leopards and perhaps Sri Lankan leopards at Melbourne; with the opportunity to continue to see lions at Werribee - which will considerably grow in profile from 2024 onwards.
Melbourne are still of the belief international tourists will have a cry if they can’t see an abundance of natives (they clearly don’t have the confidence to shift them all to Healesville as per the reasoning above); but like you say, perhaps in a decade or two there will be a rethink as to how the collections can better compliment each other.