When someone else attacks you, you don't need to spend any Wealth or Personnel to fight back. They're risking the cost of invasion and you're risking your ownership of the district.
I don't think it needs to be more realistic. I personally like Herald's idea that there's Personnel permanently allocated to like, construction and infrastructure projects or aid programs within the district you've upgraded.
And I've been implying in all my collabs that there was a big city-wide conflict in the recent past that necessitated everyone burning their resources to stay afloat kekek.
The Wealth/Personnel thing is likely to assist in dictating the pacing of people's expansion and PvP hijinks, while the RP aspect exists in part to ensure that Wealth and Personnel alone does not become the sole reason why you're 'winning' the RP.