These are truly uncharted waters, so the situation or any ending resulting from the decision seems completely unpredictable at the moment. One thing seems certain--lawmakers will be seeking to lay blame at someone's feet for this, if chaos should ensue.
The E.U. did something very clever, perhaps more for self-preservation but they have been fairly patient and accommodating throughout this entire process, because they don't want any measure of blame if/when sh*t hits the fan for the U.K. So, even as the E.U. has been firm on the rules, they also have taken every meeting, held tireless hours of negotiations and approved extensions requests from the U.K. over the past few years.
As I posted up-thread though, the E.U. wants the U.K. to "sh*t, or get off the pot" at this point. I think the E.U. is girding for the pain of a "No Deal Brexit" and just wants to be done with Great Britain.
If Boris Johnson flops this (which there is a higher than average probability that this will happen), there is likely to be a general election for a new occupant for his current position. Remember Johnson was selected by his party but not by the British voting public, many of whom are jonesing for an opportunity to chuck Johnson out.
Another possibility, if things go badly, is Scottish independence won't seem like such a far-fetched possibility anymore. People will no longer scoff at another referendum to vote on independence.
Also, even though there was a negotiated agreement for the 'Irish backstop', if there's no agreement in place for that...well, let's just not get into that because...Northern Ireland and the Republic of Ireland truly do have a history of violent conflict, that I seriously doubt anyone wants to revisit.