It is not the hardening of the skin alone. Insulin is a growth factor too. If injected too often at the same location additional tissue will develop. This may look unfavourable and the absobtion rate of the insulin will often be reduced. The tissue will develop slowly and in 5 to 10 years the first changes will be visible if you do not rotate properly. In my experience you will not loose the location for injections. You just use it less often. The more distributed the injections are the better the skin can recover - even from these long term skin changes. For pumpers this is not that easy to say. It seems that the permanent injection of a needle and the constant flow of insulin and its preservatives causes more damage than shots.
I guess that makes sense. We weren’t designed to have foreign objects permanently inserted in us…