This is surely true no matter how strong the encryption scheme, though. If you share your content with anyone else, then your security is only as strong as theirs.
You have to explicitly enable it.
Apple's documentation needs to be a little clearer on this but the system is basically designed with the accurate notion that the biggest footgun for 90% of users is not "the government was able to access my messages with a warrant" but rather "I lost access to my end to end encrypted messages".
Bit of a broad stroke anyway.
If it's the latter then yes, you can (almost) always choose not to use a service or website. That's a poor argument to defend bad privacy practices.
If it's the former then just disabling iCloud backup does not result in E2E storage of other iCloud data. iCloud photos, drive, etc. are still not E2E.