If Canada lets the military die, that will make it harder for Canada to be a force in international politics. As a result, the world community's dealing with conflict and aftermath will have less Canadian input, and that's a bad thing, though not critical to Canada, I would think.
But territorial integrity? That's just silly. If any country but the US tried to invade Canada, the US would stop them, if Canada couldn't. It doesn't matter if Canada is 1 or 2 or 6 on the friendship list, we aren't going to let a western democracy get crushed by invaders, especially as that friendly western democracy has thousands of miles of unprotected border with us, and is one of our biggest economic partners.
On the other hand, if the US decided to invade Canada (which it certainly would not), it would be very difficult for even the most ready Canadian military to resist for long. Canada would, however, certainly get aid from Europe against what would have to be a tyrannical American dictatorial regime. And Europe has, among other weapons, nukes. Canada's territory is safe.