I don’t know if I really agree that a black hole’s mass is not because it has “stuff” (that is, massive entities) inside it. How do you argue that?
In simpler terms, let’s say a star has a mass just below the limit of making it a black hole. Then, a planet collides and it crosses the threshold. Are you saying the “stuff” disappears ?
Because the solutions of the Einstein Field Equation that describe black holes are vacuum solutions. "Vacuum" means "no matter or energy".
> let’s say a star has a mass just below the limit of making it a black hole.
There is no lower limit to the mass of a black hole.
There are maximum mass limits for white dwarfs and neutron stars, and stars more massive than those limits (after they have undergone supernova explosions and probably shed a large portion of their original mass) will collapse to black holes.
As for what happens to the matter in the original object that collapses to a black hole, it reaches the singularity at the center of the hole and is destroyed. But anyone falling into the hole after the collapse will never encounter that collapsing matter; all they will see is vacuum.
We don’t actually know what happens to the matter when it reaches the singularity. Having a singularity effectively means “we don’t know what happens there”.
More precisely, physicists believe that the presence of the singularity in this solution of the Einstein Field Equation means that GR breaks down there and we will need some other theory (the best candidate at this point appears to be a quantum gravity theory) to figure out what happens there. But unless and until we discover that other theory, GR is the best we have and the best we can do is to describe what it predicts and acknowledge the limitations.
> I’d argue that GR literally provides no prediction for what happens at the singularity
That's false; GR does make a definite prediction: that curvature invariants increase without bound as the singularity is approached, but that they are finite everywhere in the actual spacetime (see further comments below). (Actually it's more nuanced than that; there are cases where there are incomplete geodesics, which is how "singularity" is actually defined in GR, but not unbounded invariants. But those are edge cases that aren't relevant to what we're discussing here.)
The Stack Exchange thread's claim about geodesic incompleteness is misleading. The actual singularity--heuristically, the "point where things become infinite"--is not included in the spacetime manifold. It is an abstract "boundary point" that is not part of spacetime. What is actually included in the manifold is perfectly well-defined, with nothing infinite anywhere, and is a perfectly self-consistent mathematical model that makes perfectly valid predictions for everything in it.
The Stack Exchange commenter basically doesn't like the fact that GR says "sorry, your model ends here, you can't extend it any further", which is fine, but it's not the same as saying GR must be wrong when it says that. It's just something many physicists are uncomfortable with and so they are looking for a model that doesn't have that property, like quantum gravity. But that's no guarantee that they will find one, nor is it a guarantee that GR must end up being overridden in this regime. It's just a best guess of many physicists at our current state of knowledge. Nor does it mean that GR doesn't make definite predictions; it just means GR's predictions are ones that most physicists would not like having to be stuck with if that's how it turns out. But nature doesn't care what humans like or don't like.
> gravity propagates at the speed of light... And light is not fast enough to escape the black hole.
Gravity doesn't have to get out of the hole. The gravity you feel outside a black hole is due to the global spacetime geometry, not to any "force" coming out of the hole. If you want to attribute it ultimately to the presence of matter, it is the matter in your past light cone, which originally collapsed to form the hole, before it fell below the event horizon.
> Light is “fast enough” it just redshifts to nothing. Gravity has no equivalent
This is wrong in two ways. First, light at the hole's horizon does not "redshift to nothing"; it just stays at the same radial coordinate because of the curvature of spacetime. The "redshift to nothing" view is an illusion, created by a bad choice of coordinates; that illusion was corrected in the late 1950s and early 1960s by the discovery of better coordinates.
Second, gravity does have an equivalent to light: gravitational radiation. Gravitational waves travel on null geodesics, just like light, and any gravitational waves at the hole's horizon would stay at the same radial coordinate just as light does.
Let’s say there is a light bulb inside the event horizon. It emits a photon. From the light bulb’s perspective, it sees the photon speeding away at the speed of light.
From an outside observer, we never see the photon. It can’t make it out. But — the photon does exist, and it is traveling at the speed of light over a finite (though deeply warped in spacetime) distance. How does the photon not reach us then? Is it not going at the speed of light?
The redshift model applies here; the photon is redshifted until it has no energy from our frame.
In simpler terms, let’s say a star has a mass just below the limit of making it a black hole. Then, a planet collides and it crosses the threshold. Are you saying the “stuff” disappears ?