> It's major refactoring when so much stuff directly includes 3rd party CSS and script, or just injects static CSS and JS in to pages inline.

You can still use CSP, and whitelist specific third-party domains. (There's no way to whitelist inline code, though, and if there were it'd be more work than eliminating it and moving it to files.)

But the refactoring is well worth doing. By eliminating inline JavaScript and off-domain JavaScript, you eliminate the possibility of script injection. And directly including third-party JavaScript (or CSS, which can include JavaScript) opens you up to various kinds of attacks.