Complements are obligatory elements and they complete the meaning of a subject or object that they relate to. Without them, a SV(O)C sentence would not make sense, or would not convey the essence of the intended meaning (incomplete meaning). 'to Stockholm' does not complete anything: without it, the sentence is already complete. 'to Stockholm' gives extra info.
1. They elected him president. SVOC
2. They elected him
president. SVO (president = he (by election))
C is needed because without it the sentence would not convey the basic idea. 1. ≠ 2.
We flew to Stockholm. SV
We flew. SV
'to Stockholm' does not contribute to the core idea of the sentence. It only adds extra information.