Lots of people say this. Is it really how the live politics would work if, say, 330 MPs wanted to vote down the deal & did so? It would not be so static
If an anti-deal majority of MPs had entirely conflicting aims, then it might be the case (eg if 165 were still-Remainers and 165 wanted a harder Brexit) that no deal would be the result
IF (nb: this is a very big entirely hypothetical if) an anti-deal majority of MPs had another plan, they could secure it IF (another if, but surely not implausible) other actors - eg EU27 - were engaged with an anti-deal majority
Eg, IF EU27 said 'we'd put EEA back on the table if there is a UK majority for it' or 'you can have a 6 month extension to A50 if UK Commons had voted for a referendum' *and* 350 MPs wanted these, then politically could eg direct govt to extend A50, hold vote etc
The problem here is a *political* one - lack of majority support for an alternative plan (such as an A50 extension, or a referendum). But it is not necessarily procedurally impossible to reopen IF such a majority were to exist (& IF EU27 wants to move too)
Basically, this is Parliamentary Sovereignty. If there is a settled majority in Parliament, that majority can find a way to change the UK govt's policy towards any issue if it wants to insist on doing so, whatever the fallout for the government.
But Parliamentary Sovereignty can not help to achieve something that does not have sufficient (majority) support in Parliament, so it would depend on what the MPs decide to do, or not, if these circumstances arose.
These are all very big reasons to do the transition deal that is available, This is what the 2017GE result really changed: this just doesn't look like the sort of government which could come out of a No Deal Crisis stronger!
