There are several possible conclusions of the Brexit negotiations. This could all could finish with no deal, or with acceptance of the deal that Parliament has already objected to. A second referendum might even finish with the UK deciding to stay in the EU, but that is unlikely.
The fundamental problem with the existing offer is a simple one: it is incomplete. Article 50 made provision for both a withdrawal agreement and an agreement about the future relationship. The second part is missing. There is a “political declaration”, but there isn’t a legally binding agreement about the future. And that’s why the withdrawal agreement had to come with a ‘backstop’. The backstop is only necessary because nothing has been firmly decided about the future.
We arrived at this situation through a combination of ill-considered procedural decisions. The EU should not have insisted on postponing discussion of the future relationship until after the withdrawal agreement had been negotiated; that was inconsistent with its treaty obligations. The UK government should not have consented to the timetable. Nevertheless, that is what happened. The way out of the dilemma now is to conclude the unfinished decisions about the future – in other words, negotiating the trade agreement that should have been on the table two years ago.
It might not be possible to make this agreement in the remaining time; that argues for an extension of the notice period. But a final agreement would not require anyone to revisit the withdrawal agreement, it would have the advantage of saving face for both the EU and the UK government, and it would avoid a situation which none of the parties wants.