This problem is due to the type of material used. The soft grips are molded from TPE - ThermoPlastic Elastomer - a mixture of a thermoplastic and rubber. This stuff can be molded in a few seconds vs. several minutes that rubber takes. So for a mass-produced product, TPE is an amazing space-age material. You can alter the softness of the material by changing the ratio of plastic to rubber. Google "Santoprene" for more information.
But rubber contains oil, and the more rubber you have in a TPE, the more oil you have. Over time, the oil can migrate from the rubber to the adhesive. This can cause it to come loose. If you use a harder TPE like the low-end Nikons and Canons, it will adhere better, but you lose the soft grip. Anything that would not come loose would not be removable for service. So you have to decide between soft grip or good adhesion. I'll take the soft grip.
Heat and solvents are the enemy of the adhesive. Leaving the camera in a hot car will soften the adhesive, and if you start using it right away, you cause it to "slip" and tear the bond. Some of these new anti-bacterial hand cleaners contain alcohol and will degrade the grip. Storage with bottles of alcohol-based lens and sensor cleaners will cause degradation of the adhesive.
I am hopeful that Nikon will come up with a better adhesive, and I can tell you the TPE vendors are well aware of this issue and are working on it. So this problem should go away at some point.
BTW, plastic bodies could be made with 2-shot molding. That is where you mold the rigid part first, then mold the soft TPE on top. The second shot injects hot TPE on the rigid part, which melts the surface, causing it to bond. This allows them to have both a soft grip and a rigid support structure. This is how they mold consumer products like screwdriver handles, power tools and other plastic parts with soft grips. But when you have a metal body (Nikon pro camera), it can be tough to get the soft TPE to adhere.