The problem is that the people who are going to be faced with crossing a border to get to work, see their friends and family, visit a doctor, take their animals to market, transport their milk ... are not in favour of Brexit and will resoundingly reject Johnson's border proposal.
Most people will reject it peacefully, but it only takes a small number of dissidents to start removing infrastructure and targeting contractors, and then you'll need police to protect those installations and those workers, and then (when the police get targeted) the army too.
Can you see where this is going? There is a thread on twitter this morning about people's memories of the Troubles:
"Halloween 1993. UFF entered Rising Sun Bar & shouted “Trick or Treat” before opening fire. 7 people died that night, 1 later.
^On the monday a school bus pulled up by ours with a sign in the window:
“Trick or Treat. 7-0”
Kids, gloating about mass murder.
We can’t go back to that.^"
"One of my earliest memories is of the aftermath of the Narrow Water bomb and hearing helicopters flying everywhere, shortly after that I watched my Dad give his car keys to an IRA man who was pointing a gun at my Mum, presumably so they could use his car to move weapons^"
"^My Mum taught at one of the military primary schools and was issued, like all the civilian staff, a mirror on a stick to check under her car every morning, trip to the shops... - any time she hadn't parked at the school (which was inside the fence). The GFA is vital."