What has been done by a referendum can only be undone by a referendum. That the first was marred by illegality with Putin putting his thumb on the scales for leave strengthens the case for another referendum but doesn’t change that to simply pretend it never happened would be undemocratic.
On top of that, just canceling would open the way for a future Bexit PM simply pulling us out to the EU without a referendum. The ERG have one third of the Tory parliamentary party - that is all they need to get their candidate into the final round of a leadership contest. The majority of the Tory membership are already backing a no deal Brexit. So the next leader of the Tory party is going to be a hardline NoDeal Brexiteer. As Britain has a First Past the Post there is every chance that eventually they will get their person into Number 10. However extreme the Tories become, the tendency of supporters to stick with their traditional party along with the way in a two party system that people will vote for one of the two parties to punish one of the parties despite misgivings about the other - Trump’s election should be proof of that.
And if that happens, by canceling Article 50 without a referendum we will have made their aim of dragging us out of the EU so much easier.