Its because there is no solution.
These are the options:
Island of Ireland stays in customs union
UK stay in customs union
Accept a backstop
Hard border
You forgot to mention the CTA (common travel area) and that really is not going to go away, coupled with a digital border.
Another alternative of course is that the Republic follow us out, they will eventually anyway so why not make it sooner rather than later, they all know Varadkars overplayed his hand.
Notch, who's paying you? who are you working for?