In theory if tombstones are to last forever, the whole world would eventually become graveyards.
Graveyards are hardly subsidized, they are for-profit businesses, and even if they run out of space, could survive through two major depressions before bankruptcy.
How much in land taxes do they pay?
Estimates are for 100 billion humans have ever lived, plus (currently) 255 human births per minute (and falling) = 331,017 years
One of the oldest cemeteries in my town has dates going as far back as mid-1800s and I don’t think they go through periodically and dig bodies up.