Certainly anyone promoting this as a policy goal would brush over the fact that you stated, and exagerate the benefits. However I doubt everybody owning a handgun would go out and buy a rifle. There is also the likelihood that many situations involving the use of handguns would not eventuate if the person in question had a rifle instead.
But again, I'm not actually in favour of this anyway.
But again, I'm not actually in favour of this anyway.