Concentration is the problem, here. You only get one concentration, and devoting that to a defensive spell is a hard sell, often. People want that concentration for offensive magic. Thus, spells like this often fall too far down the priority list to be used. If you have access to it, a 4th level Armor of Agathys is going to be highly prefereable to a Stoneskin because of the lack of concentration. It may only mitigate 20 damage, but the lack of concentration is huge (as is the melee counterresponse damage).
If they want defensive spells to matter in the next edition, they need to get rid of concentration for them. Instead, give them short durations of 3 rounds plus 1 round per level of 'upcasting' and bonus action casting times. If stoneskin were a bonus action to cast and lasted 3 rounds at 4th level and 4 at 5th ... then it would get more use.