Might as well just throw some common sense out there:
Uchiha clan were known to make sacrifices to learn the secret of the Sharingan, which is something Itachi mentions to Sasuke during their first encounter before the battle commenced. This would explain the stash of Sharingans with Madara in his cave and Obito's knowledge in maintaining a large stash taken after the Uchiha massacre. Itachi again mentioned the factors behind the over-usage of Izanagi. As Izanagi results in loss of light in the eye - like Izanami - it was done so to record successful encounters for future battle references, as a Sharingan is a copy wheel after all.
This is where the spare Sharingans from fallen comrades would come in, as I doubt a clan known for its eyes wouldn't have contingency plans in place to utilize the collected battle information through Izanagi and Izanami even. This fits well with Itachi's mentions of Clan's sacrifices to learn the Doujutsu secrets for higher tier Sharingans, and a stash of Sharingans with Madara. Coming back to the topic, an anthropological aspect is very important.
People should remember that Sasuke is Itachi's brother and as tools are passed on between clan members - Sharingans given the context and their usage by fellow clan members - Itachi's eyes are his legacy, as he's the last living relative. Getting a rightful ownership of the eyes isn't stealing or borrowing, as per clan rule - even in real life, tools, folklores, techniques etc are passed on through generations within a family or tightly knitted community made up of close familial ties - Itachi's eyes after his death are Sasuke's - as he's his brother and his family member.
I agree with OP.
