I'm leaving that part out, because it obfuscates things. In one interpretation "Authorized Participants" on the exchanges keep the number of available "coupons

I'm leaving that part out, because it obfuscates things. In one interpretation "Authorized Participants" on the exchanges keep the number of available "coupons"/shares at a fixed number. So the market depth for any ETF should be constant, regardless of the value of the underlying assets (assuming non-extreme values and outside of crashes or rapid movements). And yes, to be more exact, the market depth for an ETF should be a function of the amount of trading occuring in that ETF, not so much a constant. In any reasonable timeframe it should be constant.