Peace be upon you and God's mercy and blessings.
We know and affirm that the scholars, may God protect them, only issue fatwas (legal opinions) based on evidence. Therefore, in the matter of khums, they have definitive proof for their stance on its obligation. The evidence is not limited to Quranic verses, as is well known, since many details of Islamic law have been derived from the narrations of the Prophet and his family, peace be upon them. This response is not intended to detail the evidence relied upon by the scholars.
However, in brief, we can say:
First: The term 'ghanimah' (booty) linguistically refers to anything a person gains or profits from, and is not restricted to war booty alone. The fact that the verse is in the context of war does not mean it is exclusively about war booty, as is clear to specialists.
Second: There are narrations, such as one from Imam Abu Ja'far al-Sani (peace be upon him), where some of his companions wrote to him about khums and asked: "Inform me about khums. Is it applicable to everything a man benefits from, whether little or much, from all types of income and crafts, and how is that?" He replied in writing, "Khums is after expenses." If khums were not obligatory on everything mentioned by the questioner, the Imam would have clarified that instead of merely explaining how khums is calculated after expenses.
May you be successful in all good.
Source:
(1) Wasail al-Shi'a, Vol. 9, p. 500