Here's how it could work: For every week that an nation maintains control over a region uncontested, the daily payouts for that region would decrease by a fractional multiplier. For instance, starting at 1x, the multiplier could decrease by increments such as 0.1x or 0.2x each week, going to at-most 0.5x.
It's essential to consider the economic implications of unchallenged dominance. Currently, every payout injects a significant influx of resources and currency into the server economy. However, when these rewards are amassed by an alliance simply because others are too weak to contest them, it can lead to an imbalance since constantly money is being pushed into their pockets with 100% profit (in terms of costs required to contest the region every assault window).
TLDR: region payouts (i.e. the resources and the money) are quite significant, implement a progressive multiplier system for daily payouts in controlled regions on on the server will gradually reduce rewards for dominating alliances, ensuring fair gameplay experience. Additionally, it will prevent undeserved economic influxes.