I think this is a non-issue these days.
Early rims were made of steel, and had a consistent thickness which was pressed into a U shape, and then rolled into a long coil. The only parts of the rim which were thicker were the bead seat, where the tyre hooks on, and the folded corners which would thicken up some during folding.
Modern rims are made of aluminium - there are few-to-no steel rims sold these days for bicycles.
These are made from extusions of aluminium which is coiled while formed. The cross section is part of the design, and there are thicker parts where needed, and voids where it helps to save weight.
A steel rim would be like the left-most image, but without the two holes.
So there is additional support in aluminium rims that steel tended to lack.
Additionally, the valve stem itself is part of the support material. It is a small tube of brass mostly, which will resist crushing forces, and (as long as its not a sloppy fit in the hole) will perform a similar function to a reinforcing grommit.
Carbon Fibre rims are more "designed" and will have sufficient extra material in the rim at the valve hole. They are generally a lot deeper, which by-itself adds support.
So why are grommited spoke holes desirable still, where a grommited/reinforced valve hole isn't common? I suspect that the nipple in the hole still has a lot of room to move, to allow for hubs of different sized and widths.
Additionally the spokes and nipples hold your weight under a continuously-varying level of tension and less-tension, whereas the valve only has to resist the fairly-consistent pressure from inside the tube/tyre.