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The altitude of the "ground" varies by thousands of meters:

https://www.google.com/search?q=highest+airport+altitude

Although, I guess you could just check if the landing gear is up. I'm sure there are corner cases I can't think of, but it seems like that would pretty much cover it.

Many crucial functions rely on knowing if the aircraft is airborne or not. Weight-on-wheel sensors determine this.

Doesn't really solve the problem; a malicious actor could just lower the landing gear and then turn the transponder off.

Whatever, the point is, there should be a way to determine if a plane is in flight, and then disable the ability to turn off the transponder.

While this makes sense it may have to do what the poster above you said

Lastly, I believe all electronics on the plane must be on a breaker circuit in case of electrical shorts so that pilots :can isolate and turn off faulty circuits.

What if the altitude meter breaks? Does the transponder go off?