The size of the plot and headstone. The common use of heavyweight steel for coffins that can delay decomposition even longer. Concrete grave linings. Cremation is no better from an impact perspective.
I think they may be aiming for corpses that hang around as long as ancient Egypt achieved. :)
Ever been to cemeteries in Paris? Giant monuments, incredibly ornate graves.. The US is far more modest by comparison.
In general, humanity has buried and burned people with expensive objects to show reverence or recognize status. The Egyptians, Greeks, Romans, etc. It's kind of a tradition to waste a bunch of money on the dead!
Composting provides extra oxygen and whatever other steps are necessary to accelerate the decomposition process.
A buried body can last many years, especially if it’s several feet into the clay layer.
Try it. Bury a cat or dog six feet under and come back in five years. If it’s common clay, a large percentage will still be there.
And in some cases (e.g. peat) "many" can end up being "hundreds" or even "thousands".
I imagine it will be at least that long, and likely much longer, until the land used for cemeteries is repurposed.
But I tend to agree that composting is a bit... strange.