I would counter that although there may be some additional costs. The Full M Lok or Keymod rail slots would still allow a less cheese grater grip, with all the options of mounts well giving more ventilation for heat and they would also double as lightening cuts. So the rail is a little lighter making the rifle just a little lighter.
Full m Lok or full key mod are definitely less cheese grater than quad rails, however I think they are more cheese grater than having a smooth handguard.
Of course full m Lok or full key mod offers the benefits of being able to accommodate accessories easily on the whole handguard straight odd the bat, but your average infantryman probably won't need to do that. Add on the costs of manufacturing full m Lok or full key mod Vs a smooth handguard as well as the greater comfort of a smooth handguard Vs full m Lok/key mod, and I think they chose a good compromise with the handguard design.
12 o'clock rail, with smooth handguard at 3, 9 and presumably 6, but with 3, 6 and 9 also being able to accommodate short rail pieces.
Seems like a very reasonable and practical compromise for a wide issue handguard.
I fully expect more accessorised handguard options to emerge for certain users like SOF, which could include full m Lok or maybe even quad rails. (The DMR variant seems to have top rail with full m Lok, incidentally).
But as a baseline wide issue design, I think the handguard design they went with is a very good compromise and gives up very little while gaining a fair bit.