I do not think your conclusion necessarily follows. You are not controlling for target numbers and thus opportunity. In an office building where one has to go room to room it takes time to find targets, it is not as target rich an environment as say a movie theater, concert venue, or a night club, for instance. With a semi automatic handgun in each hand a shooter could do a world of damage in a such target rich environments. Still, in the end it is a bit of a moot point. There are different and very effective killing tools for nearly any situation. This is probably not a one platform problem, AR 15 or handgun. This is a problem of conception, that folks need to die for the pain of an individual so that the individual's pain can be appreciated. When did we come to a state where this is how individuals share their pain? Why do they think they will be taken seriously only if they act out in this manner? Me, I have no answers.
Certainly the AR 15 is popular because it is capable. So too other weapon platforms.
Had this most recent shooter killed a few more, just a few more, establishing a record, would that really change the talk form one weapon platform to another? I think you are focusing on happenstance, a bit. No problem, I get your point, AR 15 have been used in some famous shootings recently. They will be again, I all likelihood. So too handguns and probably shotguns as well. I get the desire to legislate against depravity of heart and mind. It is a noble effort but one probably doomed to failure. Still, the effort at doing something is, I suppose, an expression of hope, a desire for change, and those are necessary to effectuate change. Keep up the fight but do not be rigid in solutions sought, do not confuse issues due to happenstance.