Whilst at first glance some kind of average national room rate sounds equitable in principle for setting a proportionate threshold by nationality or residence, it kind of also assumes customers never travel overseas where they could quickly ramp up (or slow down) their spend disportionately. Bonvoy may also then see a sudden rapid growth of new members claiming to be resident in wherever is cheapest. No doubt this forum would see endless arbitrage threads.

I think it does need some kind of standardised measure. A currency amount is one option, although even the relative value of that varies with exchange rate movements. Anything based on money will likely have some inequalities. Another option might be to use actual nights spent in hotel beds per year, ignoring EQNs from promos and credit cards. That might actually be a better measure of loyalty but would run contrary to the apparent broader strategy to give away lower levels of status without having to stay in hotels. It's not insurmountable though, and I'm sure there are more alternatives people could come up with.