Why no major navy should build only battlecruisers

Which country would benefit most from replacing its battlefleet with battlecruisers?

None should do so exclusively. Japan could make the strongest case for a fast-capital-ship emphasis, but it would still need battleworthy ships and a balanced supporting fleet.

A navy composed entirely of battlecruisers would pay capital-ship prices for missions that smaller vessels could perform better. It would lack the cruisers, destroyers, escorts, submarines, and auxiliaries needed to find enemies and sustain operations. Japan comes closest to a plausible emphasis on fast capital ships because a spoiler strategy values concentration and speed. Even there, the ships must be battleworthy fast battleships and operate within a balanced force rather than replace every other type.