You're operating under the assumption that if this is turned on by default in the future, everything will go to CloudFlare. While CloudFlare is being used for

You're operating under the assumption that if this is turned on by default in the future, everything will go to CloudFlare. While CloudFlare is being used for the opt-in study, you have no evidence that they will be used in any on-by-default scenario. Nor do you have any evidence that only a single DoH provider will be used globally. Perhaps you should monitor Mozilla mailing lists.