Really? In your simplistic view, these are the only two scenarios which could play out? Let me offer a few more for you.
1) He/She never actually returns to Canada to live, but continues to reside overseas. However, the father falls ill and needs a heart bypass. They decide to temporarily return to Canada, use the public health system and then once things are better, return to their lives overseas. Yes, it's healthcare fraud but they could always say that it was their intention to live in Canada, but they changed their minds. They get the benefit while leaving taxpayers with the bill.
2) The family never returns to Canada to live. However, the kids (and the kids of the kids) all attend public school and/or university at lower tuition rates. Canada never benefits but has to pick up the bill again.
3) The family continues to live overseas where a war breaks out. They rely upon the Canadian government to evacuate them to Canada. Once teh war is over, they return back to where they were living. Canadian taxpayers pick up the tab.
All three of these have occurred repeatedly already, and are not hypothetical situations.
So, in your perfect world, there may not be any opportunity for abuse. However, in real life, we have already seen that this is not the case.