“Really? Terrorist have used that border to enter the US. But that doesn’t really matter does it?”
The only ones that I know of came in legally through that border. When they start sneaking in, or when (and if) Canada decides to be a conduit for Mexican other people entering this country, then it will be time for a wall.
It’s a legitimate issue to review security along the northern border and beef it up where and how it is needed. Probably not a wall, but perhaps fencing or even a small wall might be in order in some places.