Redundant systems are almost always good in aviation. The ICAO report[0] indicated either a system failure or user error resulted in the A/P’s INS mode failing to activate and instead remaining in HDG mode.

Yes, the old systems are effective. A lowly candidate private pilot must demonstrate proficient VOR navigation to obtain a certificate — not sure about light sport, recreational, or gliders — along with commercial pilots and airline transport pilots (ATPs). GPS is not mandatory equipment. Even if so equipped, the GPS can fail.

Yes, at least some of the redundant ground-based radio navaids should remain in place as backups.

[0]: https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007#In...

My father got an 18 metre 1970s-vintage yacht in 1994 or thereabouts. I remember learning to plot accurate fixes using LORAN signals and hyperbolic TD lines. Getting good at that and coming up with (what I thought) were really clever transformations between Euclidean maps with hyperbolic curves to straight lines on a curved surface served me very well (at least in terms of intuition) when later I learnt of non-Euclidean geometries.

(I also remember looking through ancient manuals and building up an intuition for troubleshooting and fixing a thermionic valve-based RADAR, at least once in high seas with almost zero visibility as howling winds and lashing rain was pushing us towards a rock face. Fun times.)

No - it's more of an issue that there should two tools under US control, so that if one goes down, life goes on and nobody dies.

But there's now glonass (russian gps) and galileo (eu gps) and I just learned that India and Japan have systems. I want my receivers to use as many of those as possible. they can go out and be spoofed and when the next big one comes, who knows what will be working.

Civilian GPS was also intentionally heavily degraded until 2000. https://www.gps.gov/systems/gps/modernization/sa/

On top of that: isn't the solution for USAF to stop jamming a widely civilian used protocol? Forcing others to keep using old tools and not realy on newer and better tools just so they can jam it at will seems counterintuitive

The Air Force is jamming GPS. What if an adversary did?

Civilian equipment is often not prepared for dealing with an unexpected war. I don't think this is a problem to be solved.

There's still thousands of planes in the sky whose safety needs to be considered in such a (predictable) event.

It's not a civilian protocol - it's run by Air Force Space Command (the 2nd Space Operations Squadron) primarily for the benefit of the military, but as a matter of policy, the DoD also makes it available for civilian use. The key point is that military use comes first.