Besides "access" to the PRS/ non-public keys, there are operational considerations as well. Which are generally more aligned with the reasons "redundant" GNSS systems were invested in by other countries(/EU), even though NAVSTAR GPS (US) has nominally free, worldwide civilian access, in addition to the open nature of Galileo mentioned by GP.
Because the civilian aspects of it has become so critically important and relied upon, we tend to forget that GNSS is primarily a military program/tool/weapon, which for NAVSTAR, is still under US Airforce custody and control.
We, as John Q Public, just aren't generally aware that military capabilities go far beyond a more precise location fix , since the other non-public considerations:
1)are highly technical "how the sausage gets made" decisions and details, including how the [geodetic] network adjustments are made, dictating things like leap seconds, the internal coordinate system's x,y,z origin point & orientation, and grid movement with relation to plate tectonics, etc),
2) are back-end, non public facing, or secondary capabilities (ie emergency beacon/ EPIRB monitoring, signals intelligence gathering, ballistic missile launch/ ordinance explosion/ nuclear blast detection, or measuring deviation the sat vehicles' orbital parameters [and atomic clock relativistic shifts, and suspected absolute gravity measurements] to develop a precise geoid model- for aiming icbms, modeling missile defense, and navigating and/or detecting nuclear submarines) or
3) are wartime capabilities that are turned off normally, but require "admin" access to implement, like selective availability (SA) type degradation/ spoofing, turning off civilian bands/signals (either locally or worldwide), controlling signal strength and anti-jamming capabilities, etc. This would also include things like controlling spare sat vehicles, and determining what areas get 100% coverage in the event of a less than 100% coverage sat network.
All of these aspects affect various military operations, and any country(/EU) that aspires to be a world superpower will not be able to rely on third parties to get all the military benefits of operating such a system.
Also worth mentioning that civilian NAVSTAR GPS recievers have both an altitude and speed limit imposed on them client-side, so (over simplifying things here) foreign parties looking for space or missile guidance based on GPS have always -theoretically- needed to re-engineer/re-invent a client receiver if they wanted such capability.
Somewhat ironically, or at least counterintuitively, the US military specifically avoids relying on GNSS for navigating ICBMs, smart-bombs, cruise missiles, or any other systems that must operate in contested airspace, specifically because they are aware both of how fragile the system can be (signal strength is already well below the noise levels), and how easy it is to purposefully turn off at will.
Consider that in the event of a major war, NAVSTAR GPS is more likely to be turned off over the US than over enemy territory, as we wouldn't want to help the enemy navigate or locate targets.