I can see the thinking but realistically how many 'visitors' are going to make the special journey up to the Zoo from St Helier or wherever but not want to actually go in? I can't imagine it would be very many.
St. Helier is only 15 minutes drive away via good main roads.
You'd be suprised!
In Jersey, the activity of choice on the weekend is a drive/cycle/walk/busride out to somewhere nice to have a bacon roll/coffee/breakfast/light lunch with friends, before going for a coastal walk, to the beach or visiting another attraction. (by "attraction" I mean "Jersey Potteries", "The Shell Garden", "The Hungry Man" at Rozel and anywhere you can see some Jersey cows!!!)
Trinity is a gorgeous part of the island, with very little in the way of facilities or a nice place for a coffee with friends, we're 5 minutes drive from the coast and some of my favourite beaches, 10 minutes from Gorey Castle, less than that from St Catherines Woods (the only sizable woodland on the island!).
I know it probably sounds bizarre to you mainlanders!!! But we don't have Alton towers here, we don't have any shopping malls, we don't have those kinds of things

So people find their entertainment in more quaint ways.
You can't solve a flagging tourism industry, but you can find ways to make your local populus happier and encouraged to visit for other reasons.
The right entrance is a critical factor for any successful zoo/wildlife park etc.
Somebody notable once said, you could have one scrappy lion in a corner, so long as the toilets, the cafe and the shop were right people will still come.
You can't underestimate the value of good facilities and presentation (obviously without any compromise to the core purpose)