If they are not going to breed the rhino, £2.8 m is a lot of money that could have been spent on new exhibits for any number of new species and the non-breeding group could've stayed where they are.
I'm not sure where the idea that they are not planning to breed from the rhino? Unless I've missed something? I had read the new housing could accommodate up to six individuals, which suggested to me they would be planning on holding a breeding group, and likely to bring in new cows to breed or additional males to stimulate breeding behaviour. It sounds to me like they are investing in their rhino for the long-term.
As for the existing group (is it two cows currently, or is the bull still alive?), the current accommodation is not up to EEP standards, so expenditure would have been required at some point anyway, as these are (I believe) EEP and not Marwell-owned animals, and would surely be asked to send them elsewhere by the EEP at some point if Marwell left them in the current housing....at which point they would then be going out of rhino too, and likely have little or no chance of going back into any rhino species. Perhaps its possible they could have been refused new rhino until an upgrade has been completed.
However, given the plans also make reference to the development being intended to help reverse declining visitor numbers, I agree with you that the elephant (or absence of one, as it were) in the room is the reduction in individuals and species within the park, and it seems strange for Marwell to have failed to incorporate a new species into the plans. I do think that the majority of new ungulate species currently causing a stir on the Whipsnade thread are of 'Least Concern' so, if it is true that Marwell's collection planning is focused only on endangered species, that greatly reduces their options for African antelope available to them that aren't already part of their collection, and seems likely that more of their existing stock will leave.
I wonder what data Marwell have collected to assess both why their visitor numbers are declining, and how they have arrived at the conclusion that the proposed development will increase them. I can only assume they are suffering the wetter summers in recent years, as zoo visitors begin to expect to have to brave the conditions less and less.