Personally, it’s hard to argue against this comment I’m afraid, and you can add Polar bear, Californian sealion, Black rhino, American bison and (looking forward) Chimpanzee to this list.
ZSL will have its reasons, but it’s not as if Whipsnade isn’t short on space!
It's difficult to disagree about how much space there is at Whipsnade, but I certainly don't think there is as much to complain about in relation to losing major species as is the case for many other zoos in the UK (e.g. Edinburgh, Marwell, Twycross).
Polar bears and Black rhinos were lost a few and a couple decades ago respectively (and Whipsnade already do well with two other rhino species), while American bison were lost more recently (I'm not sure on the exact circumstances) but there is still a small group of European bison, while this also wouldn't have been a massive loss for the visiting general public.
Californian sealion was a sizeable loss as a species, but their exhibit was not up to the standard of ZSL and it seems the zoo didn't want to make the big investment of building a brand new top notch complex while they would've also struggled to fit in to any of the main zones.
Chimps are very much still at Whipsnade, albeit an aging, non breeding group. If they were to be moved on prematurely I would imagine there would be a planned and suitable replacement. If the group was allowed to live out its days, you would like to think there would also be succession plans. In either scenario, Bonobos or a breeding group of subspecies pure chimps would perhaps be the obvious candidates, and with all due respect likely to make a better exhibit than the current group of chimps, so would hardly be a loss if that's how it was to go.
Common hippos will also be a noticeable loss for zoo nerds and the general public alike, but if the exhibit no longer suits the species as much as it once did then it is fair enough that ZSL don't want to commit to holding a breeding group in it for the medium-long term. It is good to hear the zoo already confirm that a brand new species will inhabit the space, however there are two main features of the enclosure (its' positioning in the Africa area and the two large pools) that will be difficult to be optimised simultaneously by the next inhabitant.
For making the most of the pools, my pick would be Malayan tapir, however they don't fit too well amongst the Africa zone.
In terms of making the most of the paddocks and being more geographically accurate, I would have to agree with Gelada, however a solution would have to be found for the pools (maybe as moats?).