The big animals, eg ungulates and lions are boring because they don't so much. The more interesting ones for kids especially are the nes that are very active.
Agreed that active animals are most interesting for public. But if they are active depends from enclosure design.
Example are lions in Basel vs. Arnhem. Medium-sized enclosure in Basel is richly structured, and lions are always active. Arnhem has lions in huge area of mature forest, but this is basically flat ground and tall trees. Lions sleep or stereotypically pace at the back fence waiting for food.
About animals in city zoos:
Yes, zoos need some big animals. They are most interesting for public. If zoo cares for education or fundraising, you can do it better with lions than ants.
Lots of city zoos have big, good enclosures and good breeding records. Many small city zoos in Europe have good and spacious enclosures because they have just a few.
Agreed that zoo doesn't need ALL big animals. If it has bears, apes and big cats it can go without elephants or giraffes. I naturally agree that many zoos need to get rid of their small outdated enclosures and some species of animals.
However, big animals need zoos. "Fewer animals in better conditions" is nice slogan, but breeding plans are scraped because zoo community lacks exhibit space. For example, no way all endangered subspecies of tiger or leopard or orangutan can have each self-sustaining population of 500 in zoos. WAZA said sadly, that zoos can exist without gorillas just as well as zoos exist without thylacines or wooly mammoths, but gorillas will not survive without zoos.
City zoos certainly will not be replaced by "open-range zoos" or "sanctuaries".