I think a hard border would potentially lead to a border poll and a united Ireland. It’s one way out, and possibly the only way out of having a border. The polls show support for it at the moment, which i imagine would increase with economic carnage mounting.
We can’t have the transition period (extended or not) without agreeing to the NI backstop.
No backstop=nothing else.
No backstop=catastrophic exit on the 29th