Not much point in calling for the UK to have left in June 2018 -- that is, to have immediately triggered Notification under Article 50 -- when it's now been well-established that to do so would have been unlawful. Not to mention unwise, when there was no plan for the vote going the way it did.
And therein lies the reason Cameron is to blame for this mess. He triggered a referendum on false pretences -- the main reasons were to see off the threat of Farage, kill Euroscepticism in the Tory Party, and win the 2015 election, and in two of these at least he was spectacularly unsuccessful. Then he refused to plan for either outcome. Then, when he lost that referendum, he jumped ship and left other people to work out what should happen afterwards.
If the referendum had been called honestly, then there would have been a proper plan in place for implementing the outcome for either result. And, to address the point in my first paragraph, the power to give notice under Article 50 would have been explicitly given to government when passing the Referendum Act, which would have sorted out some of the legal mess that resulted.
I could go on. But everything else Cameron achieved, for good or ill, as PM will be buried by his decision to call this referendum and then lose it.