The rarely asked yet vital question is what the EU and UK decide to do in the almost inevitable situation that a new trade agreement is not ready to be implemented by 2022 (for EU to go from talks starting to implementation in 3 years is unlikely to say the least).
Saying we will go to the backstop in this situation would potentially cut across the trade talks. And bear in mind the backstop itself is not yet fully defined, and this has to be agreed by 2020, and involves a large amount of detail.
Both sides wanted an unrealistic end date, the EU because A50 shouldn't be used for the long term relationship, the UK to pretend it would all be done by 2020, or 2022 at the latest. So if the deal is signed the clock is once again ticking...
This is one of the reasons for me why this is such an awful agreement - it may provide certainty for 21 months, and yes we need that. But it provides none thereafter, and learns nothing from the A50 process.
The EU-UK future relationship, if one document, is intended to be the biggest single agreement either side will have negotiated. The UK has barely started considering the policy implications, and the EU will have much to do as well.
Plus of course both sides are pretending we can sort out the Irish border question in the next two years when we couldn't in the last three years. Those supporting the deal as the only one bringing certainty may wish to reflect on this accidental thread.... /end
