Those won't work at all in deep wet snow on gravel because they load up and slide. You'll never find any kind of wheels on any commercial plow for a good reason.
No, actually, they work just fine. I have used them for many years.
You see, the snow in front of the plow is deep, but wheels are behind the plow, and the snow there is not.
Even if they did slide, isn't that what a skid does?
BTW: You second point is also incorrect, some of out ODOT snow plows DO have large pneumatic casters on them.
Here is a link to just one company that sells non existing caster wheels, (that you can't find), for their commercial plows.
RT Reversible Trip Blade Snow Plow