The reason warlocks gravitate to soloing is because they add very little in party play. As opposed to a wizard soloing, where arguable, the rest of the party adds very little. Every class has a capacity of soloing content, some more limited then others. Most of the soloing done on warlock does not even involve enervating shadows, it is mostly hit and run or stacking walls (which will be nerved eventually). I think you are massively overestimating the capabilities of enervation shadows, which is understandable, it also looked incredibly overpowered to me at first glance as someone who mostly plays mundane andys. 100 AC is mechanically not different then having 65 AC which many classes can achieve, the concealment is a little more reliable then 50%, but it does not make you invulnerable. If you get hit (and you will) there is no secondary layers of defense at your disposal, like Premo or other damage soaking spells and you have a small hit point pool. Overall this spell, while powerful, does not see nearly as much use in solo play as you think, aside from being a 'oh crap button'.
Imho if you think warlocks soloing content is a huge problem, give them more ways to engage and be useful in group play.