The problem faced by MPs is that nobody knows for certain - indeed, I am tempted to agree with those who argue that nobody has the least idea - what we will end up with if the deal is voted down.
I also think that a very good point was made by Anand Menon in his excellent article
"Disagree with May’s Brexit deal? Fine, but be honest about the alternatives,"
in which he points out that although the PM's proposed withdrawal agreement has many flaws, almost all of those in right, left, pro Brexit and anti-Brexit sides either have no credible alternative or are not being frank, open and realistic about the consequences of those alternatives.
For anyone who is not familiar with Schrodinger's cat, the idea is that we imagine a sealed box supplied with oxygen, cat food and water, which is impenetrable to any scanning device or form of light or radiation which might tell us what is inside, and into which we put what was, before the box was sealed, a live cat.
Also within the box is a mechanism which uses a purely random and unpredictable method to decide whether to release a poison which will quickly and painlessly kill the cat.
Schrodinger argued that if we have no means of knowing until we open the box whether the cat is alive or dead, there is a sense in which, until we actually do open it, the cat is both alive and dead as there is the potential for both states. The cat exists as a sort of wave form or probability pattern until the box is opened and the wave form "collapses" into a deal or living cat.
I have some difficulty with the idea that a cat can be both alive and dead but I have no difficulty with Schrodinger's Brexit
- E.g. we don't know until the outcome is determined whether if May's deal is voted down it will be the Brexiteers or the Remainers whose dreams are dead because there is the potential for both outcomes.
We do know, however, is that if May's deal goes down there will be one of four outcomes (I'm ignoring the question of whether there is another referendum or a general elections because these are process issues - people are putting both forward in the hope of getting the outcome they want but it is far from certain that either would solve the Brexit problem.)
Those four outcomes are
1) If parliament doesn't pass anything else by 29th March, under Article 50 Britain leaves the EU with no deal on 29th March 2019.
2) There is another round of negotiations with the EU after which a revised version of the May deal, or something similar to it, is put to parliament again and this time it passes.
3) There is a "softer" Brexit than May's deal, at least for the short term, based on EFTA or EEA membership, something similar to Norway's relationship with the EU, hence this is often referred to as the "Norway option." N.B. It is far from certain that this option is actually available.
4) Britain doesn't leave the EU after all.
Voting down the May deal is putting Brexit into the box. Only after that vote will we find out which of those four outcomes is the result. It could be a harder Brexit, a softer Brexit, or no Brexit at all.
What the outcome certainly won't be is an outcome that pleases everyone - that's not just a thought experiment, it's a cloud-cuckoo-land fantasy.
And because we won't find out what's in the box until after we open it and long after the votes have been case on Tuesday, we don't know whether Brexiteers who vote against May's deal are killing Brexit, or whether Remainers who vote against the deal are bringing about a hard Brexit.