I'm going to have to disagree with you on that. Etymology, while important, does not always capture the full range of meaning.
Res publica has much broader connotations and implications than simply the idea of a public thing. Embedded in the idea is the responsibility placed upon the citizenry as well as the concept of a shared goal by the community. Most famously, the responsibility of civic engagement was expressed in Pericles' Funeral Oration:
"We alone regard a man who takes no interest in public affairs, not as a harmless; but as a useless character."
Democracy does not necessarily have that same connotation. Democracy does not posit a shared common good that the citizenry embodies. Indeed, one can argue that a shared goal can only happen in a highly homogenous community rather than a diverse one, which democracy encourages. Moreover, until recently (i.e. about the last 150 years), democracy has been viewed with suspicion if not outright contempt. Plato's extended metaphor of the ship comes immediately to mind:
"The common citizenry trying to direct the state is analogous to sailors on a ship quarreling over the control of the helm; each thinks he ought to be steering the vessel, though he has never learnt navigation and cannot point to any teacher under whom he has served his apprenticeship; what is more, they assert that navigation is a thing that cannot be taught at all, and are ready to tear in pieces anyone who says it can.”