As a Protonmail customer, and since they're active in this thread, I think this would be an excellent place for them to reply to your concern here to say that they agree that it's not customer friendly and that they'll fix it.
The cancellation varies depending on the service. For Proton VPN for example, the cancellation now does not force downgrade you right away. But for Proton Mail, we have kept the legacy method of immediate downgrade because that service involves data storage. Because VPN has no data storage, so we can auto downgrade you at the end of the subscription to the free plan. This doesn’t work for Proton Mail because auto downgrade to free might require randomly deleting emails to fit under the free storage quota. So for that reason we ask users to actually downgrade at the time they decide to downgrade to resolve storage quota issues themselves since we cannot automatically do that later on their behalf.
:)