There are plenty of ways to avoid a WTO cliff edge in Dec 2020 without having completed an FTA - various patches can be negotiated and will be in everyone's interests. Very different from a No Deal exit
For example you could negotiate a bare bones 'Beta' trade deal covering goods, with continuity ensured in areas like energy and transport, ongoing equivalence regime for financial services, similar for data. Followed by continuing negotiation of a more comprehensive FTA
Anyone who thinks either side will contemplate a damaging "cliff edge" has learned nothing from the revealed preferences of the last year
Of course there will be some costs and frictions but that's baked in - we're leaving the single market and the customs union
(Btw for avoidance of doubt it's been clear to me for at least 2 years now that it was inevitable UK would leave both CU and SM - simply not sustainable for an economy of UK's size, with a global financial centre, to be a rule taker to that extent.)
