I took a look at xclip's code, and its scope is larger than just macOS pasteboard integration. If I volunteered to help adopt xclip into Emacs, I'd have no easy way to test its scope beyond macOS. Perhaps it should remain an external package and people just install when needed. Equivalent mac-specific pasteboard integration could live in ns-win.el and be narrower in scope just for macOS users.
As far as the proposed user option goes, "fallback" doesn't really convey any meaning to me. I know eglot adopted the convention "stay out of" to control that minor mode's enabling other minor modes. Perhaps a user option called xterm-stay-out-of-xclip, if that's a convention that people would like, or xterm-inhibit-enabling-xclip following long-standing Emacs inhibit conventions. This is moot until xclip becomes part of Emacs.