https://www.theguardian.com/politics/2017/nov/10/brexit-date...
More nuance: http://carnegieeurope.eu/strategiceurope/63955?lang=en
In short, the author of Article 50 disagrees.
Shame on them I say.
Indeed, the text is so clear, it's hard to argue that there is any actual room for disagreement:
3. The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.
The treaty will cease to apply upon a withdrawal agreement, or after 2 years, or after unanimous agreement to extend the period.
The only action that ends up with anything reasonably resembling a "reversible" Article 50 is for the UK to somehow convince all of the EU member states to unanimously agree to an unending extension. Meaning that the UK would be in a permanent state of leaving the EU.
A fun question would be whether a re-acceding country would lose its current privileges and its opt-outs of the Euro and Schengen.