I don't really understand why the EU doesn't agree to a finite end date for the back-stop and to allow the UK to unilaterally withdraw from it, given a notice period. Article 50 provides for unilateral withdrawal and a finite end date, after which there is no deal, unless a deal has been agreed. As a point of principle, a finite back-stop with unilateral withdrawal seems no different to the pre-exit position on withdrawal. An infinite back-stop with no unilateral right to withdraw puts the exiting country in a less-flexible position than pre-exit which, at a principles level, is obviously wrong.
If that concession was made, which puts the EU in no worse a position than the current situation (particularly when taken alongside the other aspects of the WA), we'd probably have a deal.
Yes, it kicks the can down the road somewhat - you've still got to deal with what happens after the back-stop - but so does "no deal" and so does the proposed WA.
I'm critical of our government's approach and ineptitude over the last few years but I think the EU's position on the back-stop is certainly a subject of valid criticism. I don't think they would be conceding much, in practical terms, by offering the unilateral ability to the UK to end it.