Why battleship guns encounter a practical ceiling

Could a navy move from 18-inch battleship guns to 24-inch weapons?

Not sensibly; eighteen inches was practical, twenty was already very difficult, and anything larger imposed excessive ship and metallurgical demands.

Gun calibre is an engineering trade rather than an automatic ladder toward larger numbers. An 18-inch weapon remained practical within known battleship design. A 20-inch gun would be far heavier and demand a much larger ship plus difficult metallurgy. Beyond twenty inches, the mounting, ammunition, structure, and production problems grow so quickly that matching a rival with more smaller guns becomes the more credible choice.