The discovery of antibiotics happened by accident while performing a completely different study, following that example we shouldn't fund these projects at all.
And you severely underestimate the number of moonshots there are. Imagine trying to find penicillin by the method you describe. There are thousands of species of mold, many of which are harmful to humans. The genus penicillium alone has over 300 species. Such a study would take decades while costing a fortune before reaching any results.
Look at it stochastically, that particular set of events, yeah random chance, super rare. But with similar behaviors, could we make similar discoveries? Hell ya!
And they all feed back on each other. Then some other idea might enable us to discover penicillin by some entirely other serendipitous route. There isn't a single path to the future.
Ridiculous like feeding people mold to see if they get sick less, or injecting people with the pus of other people to inoculate them?
The discovery of antibiotics alone has paid for all the moonshots you can fund, even if none of them work out.