OK, I'll ask again. What are the wonder slugs for?
If you say long range is not the advantage, and the rounds kick too hard to shoot a 3 shot group with, and the foster and Brenneke slugs already kill deer out to 100 yards, what is their niche? You surely don't expect anyone to trade in their .270 so they can shoot deer with a 12 gauge where rifle hunting is allowed. I've been in this game long enough to know that band wagon jumping tends to be expensive, and more often than not, the investment in a shiny new system is seldom worth the cost.
As for the earlier statement that higher velocities is what drives the gun business, I would opine that it only drives a small part of it. There has been much cynicism towards gun advertisements since the magnum craze of the '60's. Then guys began to realize that a .264 Winchester didn't do anything a .270 wouldn't do, a .300 Weatherby wouldn't do anything a .30/06 wouldn't do, and the .378 Weatherby killed game just like the .375 H&H. The real jump in velocity came with the .378 and .404 based cartridges necked down to small bore diameters, but not everyone wants one.
The important advancements in our industry since the '70's is the improvements in accuracy and bullet design, and the development of stainless steel for firearm use, synthetic stocks, and better optics and mounts. If a hunter considers his maximum range to be 300 yards in relatively open country, then having a rifle that adds 30% more velocity to the bullet doesn't change that. The long range marksman who has the skill to take game a a half mile will adjust his sight for that range, so again, the increase in velocity is not the overwhelming element responsible for his success; and again I think of the Adobe Walls shot and the civil war snipers who would leave many modern marksmen shaking their heads.