Couldn't the same arguments be made for firefighters, medical, or military personnel? Yes, yet these noble professions recognize that proper minimum staffing dramatically improves the odds of winning—and saving lives. Our question in law enforcement shouldn't be how single-officer first responders should handle active shooters. Instead, our question should be why are there so many officers working alone?
But most of us already know the answer: "We've always done it that way." Followed by: "Don't make waves."
My response to that is it's the active shooters who are making the waves. And your response to me might be, "Get real. There's no way 'they' will ever go for an idea like that." And you'd be right. "They" won't.
It's Your Call
Which puts us back to square one: the first responder on scene, deciding his or her next move. The answer depends on the situation: suspect(s), victims, location, officer(s), backup, training, departmental policy, weaponry/equipment, and "heart." While one officer might immediately move to engage, another may wait for backup.