How the Royal Navy argued for third-rate ships

Was it easier historically to justify third-rate ships because only kings and a small elite mattered?

Not as much as the premise suggests. The Royal Navy still had to persuade a sizeable and influential middle-class audience, though the media and language used to make its case changed over time.

The Royal Navy's political audience was not confined to the monarch and aristocracy. Merchants, bankers, shopkeepers, and other middle-class voters had commercial interests that made sea power and naval spending directly relevant to them. The army attracted more aristocratic attention through purchased commissions, while the navy cultivated the professional and commercial classes. Their interest in classical learning and cultivated status also helps explain the Royal Navy's use of Greek-derived ship names. Naval arguments first appeared in substantial books suited to an audience that valued learned public discussion. As the electorate and media environment changed, the service increasingly had to translate those arguments into newspaper language and memorable headlines.