I don't think it is wise to come to that conclusion so quickly. If his AR-15 hadn't jammed, this probably would have been the most deadly single-man massacre in history (he hit 70 people with a shotgun and pistols, and his AR-15 had a 100-round magazine), and I think that point merits some consideration. From what I understand of AR-15s, gun jams are somewhat common, but that isn't really a problem for adept users. It is too early to say exactly what was going on with this guy, but I get the impression that a growing insanity drove his actions more than rational planning; he probably would have been practiced if he had more of a plan as Brevick seemed to have.
It should be obvious that guns aren't going anywhere, but there is a clear distinction in policy between methods violence that affect a few people and those that affect a large number. It seems like Holmes' goal was closer to killing 100 people, and he came very close to doing so. And he obtained the means to do so without what you could even describe as a speed bump. If he had used a bomb instead, would there be an uproar over attempts to curb bomb making? I don't see why the use of a gun should make the case so easy to dismiss.