Awww we can't blame the poor old grass snakes
They only eat something the size of an adult frog once every other week Most grass snakes just love eating toads, very tasty to a grass snake, if not to many other predators. (though there is some field research indicating that some individuals are toad eaters, whilst others prefer frogs and newts!)
I think something more like this is what has happened:
The frogs are explosive breeders and have their day when ponds are fairly young. In time the number of invertebrate predators (mostly diving beetles and dragonfly larva) increase in the pond. These do for all the frog tadpoles eventually, so slowly over time the adult frogs appear to disappear as they are not being replaced.
Toads though have very distasteful (and to many predators poisonous) tadpoles, some are taken but most predators will avoid them. So in time the toads take over, the grass snakes win either way. It just wouldn't work at all for them if they were able to eat all the adult frogs from a population though, they would have become extinct thousands of years ago.