That's not true and myth perpetuated by the zoo at the time of their removal. Borders can be totally sustainable if managed correctly. For a long time the zoo grew all their own plants from seed (cost, a few pence) and selected varieties that were drought resistant.
One has to remember that the old gardeners at the zoo were highly skilled and in many ways more respected than the keepers. They didn't just go to a garden centre, randomly pick some expensive flowers and order thousands of them (as happened in recent times).
Unless you attend any National Trust property, or go to any of the RHS gardens or events, most of the NGS events or places like Cheltenham or...
Anyway, sounds like a discussion from five years ago.