Art.50 has already been worked overtime on transition (which it doesn't provide for at all), hence the growing feeling that extending the format on that legal basis alone isn't viable.
So going beyond end 2020 would mean opening up a big #MFF problem that no one wants
That points to the provisions on third-state relations, where the options are much better understood.
UK gets more space
EU avoids a cliff-edge
Problems. Of course.
The nature of the backstop means it'd be a mixed agt, so that means ratification by unanimity, inc those pesky sub-national parl'ts, so not quick or easy to do.
Plus you might be coming back to ratify the new relationship a couple of yrs later
And the big political problem for UK hard Brexiters is that they might locked in this second transition