A CNC router can carve foam blanks for a mold very easily and quickly and very accurately too. Takes very little post carving work to make a useable plug to make a glass mold. The foam for this though is pretty expensive.

The 26.48 kg of abs plastic cost $27.95/kg for a total of $740 worth of plastic alone, not to mention over 6 pounds of screws and inserts they used. A foam block about the right size seems to go for about $800.

The advantage of the 3D printing is just about all the material you bought ends up in the product, where with the foam, a large percentage ends up in the dust collector.

And I suspect the router would make a serious mess routing it, foam dust goes everywhere, worse than plaster dust.

A dust collection system is not that big a deal. The 3D printing needs a fumes extraction system, so it is about the same.

BTW, some mold producers use a CNC machine to carve an oversize mold, then the come back with a dookie-schmutz dispenser on their CNC machine that coats the foam. When this cures, they mill the surface to its final shape. Not much left to be done to have a fully functional mold.