This is a tricky issue to solve. It's not like we can region-lock domains to a specific country, like how Steam is able to use price discrimination to sell the same game for different prices in different currencies. The real price is indeed denominated in USD and billed to registrars in USD, and these prices have always remained constant for all registered domains on all of our TLDs (so indeed the real price is steadily going down over time thanks to inflation, particularly over the past year). Any other price displayed in a different currency by a registrar is performing currency conversion and is presumably subject to change in the future along with the exchange rates.
I agree with you, it doesn't seem like registrars are communicating this well. Prices of other goods (e.g. luxury watches) do also change multiple times per year to reflect changes in underlying exchange rates, but crucially, what you're getting there is a one-time purchase, and you know up front the only price you'll ever be paying for it. Domains, by contrast, are essentially multi-year subscriptions, and the price of subsequent years is liable to change both as the registrar themselves adjust pricing and as underlying currency exchange rates shift as well. I think the registrars broadly have looked at this issue and decided it would be too complicated to display the underlying USD price to registrants, so better to hide it and just display the price in the local currency?