Anyway, the competitor of iMessage and Messages is WhatsApp. Nobody is sending me SMSes except banks so even iPhone users use WhatsApp to send messages to friends and to groups in my country. If somebody would insist using only iMessage they would be out of the loop. And about Messages, well, I don't have that app, I receive no SMSes, I still communicate with everybody so I guess that nobody uses Messages too.
The issue (I think) you are referring to is that if you enable iCloud backup[2] or iCloud for Messages[3] (both of which move effectively move the storage of the messages to the cloud, either as part of the device backup or as the canonical representation that devices sync from respectively) then the messages decoded on device will be stored in blobs that iCloud has the keys to unless you enable Advanced Data Protection.
[1]: https://support.apple.com/guide/security/how-imessage-sends-...
[2]: https://support.apple.com/en-us/HT211228
[3]: https://support.apple.com/guide/icloud/what-you-can-do-with-...