why all the backup ?
because only a lunatic would base everything on one system which is not directly under their own control.
Yes, GNSS can be (and has been) spoofed, degraded and/or jammed, but there non GNSS methods which the zealots can use to guide HE where they want., not as cheaply or conveniently as GNSS, but ...
I have issues several aspects of the JCP but on the whole, ADSB is a good idea (tm).