Neither gorillas nor okapis are prone to the health problems that you see if elephants spend too much time in small barns with concrete floor. Elephants will develop foot infections and arthritis and often enough die from this diseases. To avoid this, elephants either need a lot of indoor space with sand floor so that they don`t spend time standing in their own urine, or a mild climate where they can stay outside even during the night for most of the year. Indian rhinos are actually affected by the same problems (and many european zoos have started to keep them on sand indoors as well), but most other mammals incl. both species of african rhino are not.
The point why a brought in the gorillas and okapis was because they could not use (or at least less use then eles) their outside exhibit during winter and not because of any health problems inside. But they can get faster ill/sick outside during the cold period. So they must live much longer inside in small exhibits and away from "fresh air" as the pachyderms, which some experts think is not natural/good and can cause at least mental problems.
Also, nobody has said that the indoor floor must be concrete (or concrete only). As I already wrote, spacious (and not TOO expensive) Indoor exhibits (with sand floor and bath occasions etc. - no objection) for elephants are possible in Europe. So I feel confident, that they are also in Northern USA and Canada, where there is mostly more space within a zoo area to built a big indoor exhibit. And again: If you renounce of unnecessary artificial elements, a famous architect (think at Copenhagen), touch screens and other interactive stuff and maybe even only a view from outside instead of let the visitors walking in the building, then it will not be that expensive.
In the case of Bronx Zoo: Afaik under some circumstances, White Rhinos can be affected/can suffer under the same climate problems as elephants does (e.g. frozen limbs when TOO LONG outside or catch a severe cold etc.).
To the all-time-records in attendance: Well, most of those zoos knew (fortunately) how to replace the elephants adequately. But it is no secret, that something NEW is allways drawing more interest then something you already have. In a few years, the lack of elephants will make itself felt. Kids always loved "charismatic" elephants more then most of "boring" northern animals like musk oxen, moose, deers etc. I am sure that - e.g. - many people in Chicago still missing the elephants (as the people in Frankfurt, Germany, do - decades after the last elephant left the zoo). If they could choose, they would reclaim the eles immediately.
I do not think people are missing elephants in Chicago that much because Brookfield plans on getting elephants back in the future. I have seen Many people complain about the lack of elephants on Brookfield Zoo's facebook page. However, the zoos has improved it's annual attendance every years since elephants left and has not opened a new permanent exhibit since the elephants left (and has gotten rid of some flagship species including hippos) so it does not bother them too much.
