You're just describing the ecology of parasitism/pathogens. If the fungus killed the whole colony, it would have no hosts and go extinct. A fungus that did that would therefore be selected against, as it would not be able to pass on its genes following the extermination of its hosts. The evolutionary role of the fungus isn't to keep ant colonies in check, it's to pass on its genes. As killing entire colonies quickly would be detrimental to passing on its genes, it's not something that would occur sustainably.
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You're just describing the ecology of parasitism/pathogens. If the fungus killed the whole colony, it would have no hosts and go extinct. A fungus that did that would therefore be selected against, as it would not be able to pass on its genes following the extermination of its hosts. The evolutionary role of the fungus isn't to keep ant colonies in check, it's to pass on its genes. As killing entire colonies quickly would be detrimental to passing on its genes, it's not something that would occur sustainably.