Why destroyers and light cruisers converge over time

Do destroyers become more useful and much faster as the campaign advances?

Yes; later destroyers improve substantially, and their capabilities increasingly converge with those of light cruisers.

Later destroyers become much faster and more capable. Improvements in machinery, hulls, and armament let them assume missions once reserved for larger scouts and light cruisers. The boundary between the two types consequently becomes less distinct as the campaign advances. Warship classifications describe changing bundles of capability rather than permanent technical categories.