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It's a waste to limit the central bank to just being the guardian of prices.
Literal
Central-bank [to-に] prices [genitive-の] guardian [only-だけ] do-let-keep [nominalizer-の][topic-は] wasteful.
A casual/editorial opinion. The structure ~にXだけさせておく combines the causative-ておく pattern (させておく = 'let do / leave doing') with だけ ('only') to say 'leave X doing only Y,' where the に marks the party being let do something. 物価の番人 ('guardian of prices') is a set metaphor in financial journalism for a central bank's core mandate of keeping inflation in check — the sentence's argument is that this mandate alone underutilizes the institution. もったいない, usually translated 'wasteful,' here carries its richer sense of 'too good to be limited to that' — the waste is not of money but of potential.