Zero immediate harm is done by corner crossing, but it's naive to discount how many of these landowners feel. Calling them crybabies doesn't do it justice. I'm not agreeing with or defending them, but for decades, corner crossing wasn't a concern. Unless an ambitious hunter used a plane or helicopter to access public land (which I've done twice now), the landowners controlled all the access. That was their normal, and they got used to it.
With corner crossing now being allowed, I can see how landowners are upset and fighting to maintain the status quo. It's a losing battle, and you're correct in no immediate harm being done, but many of these checkerboarded private properties will go down in value. And if you've ever known a rich person, there are two things they don't like: losing power and losing money.
The older I get, the more I realize how contentious the battle for land access is. This year, our group accessed two parcels of landlocked public land by helicopter. One ranch surrounds this land. The female landowner was so upset that she said repeatedly, "If I can't have those lands exclusively...I will go up there and destroy that land so no one can use it."
Just think about that mindset. It's insane. Yet many believe that this public land is theirs, and if they can't have sole ownership, they're willing to burn it down so no one can enjoy it. Once you understand that mindset, you realize how far someone like Eshelman (and many others) will go to maintain control of these public lands.