Consider these shorter phrases:
We are the only ones (ser)
We are in the restaurant (estar)
The verb is expressing "being" about "being the only people" which is independent of what follows (which happens to be a location). You could write the sentence in a longer form, We are the only people [who are] in the restaurant to see there are two 'being' verbs implied, in fact.
It might be tempting to still think estar is the right verb choice, since 'being the only ones' is temporary state, but, in fact, 'only' is a modifier and as such a red-herring too! Really, that sentence boils down to talking about people: We are the [optional modifer] people...
I think this is the closest rule:
I'll double check with one of our Spanish native linguists to be sure though.
Gruff's explanation is spot on. This "ser" is identifying, not talking about the position/location. Other practical examples where we use ser in this same way to identify are for example when someone knocks on the door and before you open it you ask: "¿Quién es?" (Who is it?), and the other person answers "Soy yo, Luis" (It's me, Luis). Or when you are playing a game and you win and then say "¡Yo soy el ganador!" (I am the winner!). All these are using ser to identify.
Many thanks for the quick and well explained reply!
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