Emergency braking with one hand off the bars will end in tears regardless of which hand is on the bars. Skilled riders can steer accurately with either hand and often use their less dominant hand to steer while grabbing food or drink or making adjustment to clothing and bike.
So is your suggestion safer - I believe its debatable, and even if it is, the difference is pretty marginal - how often does a rider have to pull an emergency stop while signalling, and more importantly, of those cases, how often would it make a difference to the outcome? What is less safe is riding a bike that is configured differently from what you are used to, hence the convention. If you have ever had the joy of pulling an emergency stop on a bike wired "the wrong way" you will understand what I mean.
Where I live, convention is different to where you live, however, only a few riders think enough about to worry, and fewer have a need to deal with it. Those that do learn to either adapt to new bikes quickly or reconfigure their bikes to suit them.