They only hinder the process if you treat them as demands instead of questions or comments. The questions are generally smart and thoughtful, just often mistimed/misplaced for where most companies decide to have the RFC process (i.e. often after completion rather than before starting main implementation).

It's alright to answer "No, we haven't done estimations for cloud-agnostic architecture as part of this project since it was not part of the approved goals and requirements. If we decide to go multi-cloud at some point in the future, this architecture will need to be reviewed with the rest of the infrastructure as part of the migration plan".

If that kind of answer creates a problem then the issue wasn't really to do with the RFC or the comments, that's just where the other issues in the process became apparent. Namely, the requirements are being set without all relevant stakeholders involved or even aware (which is not the same thing as agreeing).