"E U S S R Gives "solemn" Vow That Back Stop Will Be Temporary....."
That's good enough for me (he says with fingers and everything else crossed).
There's only one reason the EU want a timeless "backstop". It is so they can dictate the terms of any trade deal that might be agreed post-Brexit. They will have Northern Ireland (and to a lesser degree the whole of the UK) complying with Single Market and Customs Union rules which is, however you'd like to put it, totally contrary to the notion of the UK leaving the EU. They will have that sanction unless and until the UK acquiesces to its demands which will undoubtedly involve some sort of compliance with the Common Fisheries Policy, the Common Agricultural Policy.
However, the "backstop" arrangement is only one of a multitude of totally unacceptable conditions being imposed on the UK by Mrs May's deal. MPs are quite right to reject it. What they are not right in doing is to say that since this deal is so unacceptable the only alternative is to remain.