Should not the value of NXT decline every time a snapshot of ardor is taken?
The value is subjective, did you mean the price?
If the price declines, people will afford more NXT to get more ARDOR generating more demand for NXT, and price of NXT goes up again. It's a self-sustaining process

But where do you think will it find a balance? I'm sure not everyone will pay for a higher price in NXT just to claim Ardor.