Unless you mean ALL dreadnought style warships were obsolete by 1941 I dispute that only obsolete ships were destroyed. The USS Texas was a WWI battleship, but it still bombarded the French coast quite effectively on D-Day. Perhaps you don't think a direct and damaging assault on nearly the entire USN apart from the carriers was a threat, but the Navy then and now disagreed. Probably because even AEGIS equipped carriers unescorted today are vulnerable, but in those days lone carriers facing even light cruisers had no hope save to find them, get airborne and sink them before in range of far heavier guns. The Bismarck may have been (essentially) sunk by a carrier launched torpedo bomber, but without escort the Wolf Packs would've made sure that ship never left Bristol.

A "classically liberal" nation as you seem to envision it would've shrugged off Japans efforts to achieve and then surpass parity with the US and British navies and left itself vulnerable to far more devastating attacks that would inevitably had come from an increasingly militaristic nation that hadn't lost a war in 2000 years. Not that I think the Washington Naval Conference was properly handled, mind; it incited popular support for Japanese fascism and made war the sole remaining recourse, but isolationism is just as wrongheaded now as it was in the '30s. You don't look for trouble, but you do take all practical measures to preserve your own security.