A pre-dreadnought loophole in naval treaties

What would treaty powers do if pre-dreadnoughts were exempt and could be built without limit?

They could exploit the exemption by using pre-dreadnought-like ships to dominate the role otherwise assigned to heavy cruisers.

The premise removes pre-dreadnoughts from the limits imposed on newer capital ships. Navies would probably exploit that freedom rather than leave the category obsolete. Their armour and heavy guns would let them take over much of the heavy-cruiser role. Treaty definitions can redirect design effort toward whatever useful category remains unrestricted.