Until they figure it out and start blocking that too of course.
EDIT: or do you mean to replace "all" (content + js)?
And it's not very detectible because they do it all the time.
I own Anno 2070 (as can be seen on my steam profile), but can only play with RELOADED crack under wine because UPlay refuses to run.
Same with this type of encryption: Kazahstan can easily defeat it, but it makes it harder for people trying to debug why they can’t use Netflix (for example, in case that Kazahstan MitM's everything, and encrypts with a different certificate than your Netflix client is using).
One catch: remember that the browser itself absolutely should not be the installed program doing the end-to-end encryption, where bugs can allow the private keys to be leaked. Important data like the private keys shouldn't even be in the same address space. See gpg-agent/ssh-agent as an examples of how to keep sensitive data in a separate process.
That's one of the things that makes securing browser JS crypto so intractable.
If it's an additional source being added much later on that you are concerned with, that's always been a broken design that Douglas Crockford warned about years ago.