The problem with this is that it's universally applicable.
Any cloud service that has end-to-end encryption today can be forced to break it if the jurisdiction in which they're based passes a law requiring it.
"So use a self-hosted open-source cloud backup system with a VPS?" Not a scalable solution. I genuinely do not believe there is a scalable solution to this problem.
All we can do is either pick the service we trust will remain safe the longest, or DIY it for ourselves and maybe those closest to us. And fight at the ballot box to end the era of ever-expanding government surveillance of everyone's digital data.