Erasmus said the Springboks had been beaten by Argentina and Australia in the Rugby Championship of that year and he had decided to stand down if losing to New Zealand.
Instead, the Springboks won 36-34 and he stayed on to take the side to their World Cup success in Yokohama.
He will now step back to put more time into his job as South Africa Rugby's director of rugby.
In hindsight, that win over the All Blacks in Wellington was seen as a crucial turning point for the side where the belief in what they could achieve was kindled.
"We had a lot of small targets where we tried to simulate matches, as there was only 18 Test matches before the World Cup," Erasmus said.
"We lost to Australia and Argentina, and if we didn't win in Wellington I would have resigned.
"I have never lost three games in a row as a coach and if I did that I don't deserve to be a Springbok coach. We played New Zealand in Wellington and that was important, as if we lost it I wouldn't be here," he said after the final.
Now that the Cup is behind him he is turning his attention to the 2021 British & Irish Lions tour.
"When I took over two years, 18 months ago, it was just 618 days until the World Cup and we planned to win it. It is not about 614 days until the British & Irish Lions arrive in South Africa and we will start planning for that now," he said.
Having set a level of performance, Erasmus said the challenge was to maintain that standard.
"The big thing for us is to stay consistent. What Eddie [Jones] has done with England when they won 23 out of 24 games and was starting to be consistent and with what Warren Gatland has done with Wales – starting to be consistent – that's where we need to be.
"We were terrible at that in 2018 – we were up and down – but this year we've played 12 and lost one [and drawn one] so we have got that consistency back. And yes, we've got the World Cup here, but going into next year if you lose the first Test people forget about the World Cup and that's not where we want to be," he said.